91 ZR-1 Cutting Out Under Load
 

Haven't had a problem with this car in years until today. The car had been sitting for awhile so I decided it was time to break it out of the garage and take it for a spin. We made a 10 mile trip and the car was perfect, even while spanking a Mustang on the highway. After sitting for 30 minutes, we hopped back in and the car immediately had a problem.

At idle, it's perfect, at highway cruise, it's perfect. At any kind of load situation, it has a stuttering, cutting out problem. The heavier the load, the worse the problem is. If you put the car in 5th gear and bog it down at like 20 mph, the car literally shakes.

I'm almost positive it's not a mechanical engine problem. It seems electrical or sensor/vacuum related. The only problem I have ever had with this car is a little 5 cent clip falling off the arm of one of the secondary intake flap solonoids. When that happened, it was just down on power.

Any ideas? I'm dreading having to take this car into the Chevy dealership with a blank check.

Edit: Forgot to add, it makes no difference whether the secondary systems are engaged with the key. Either way, it has the stuttering problem.

Re: 91 ZR-1 Cutting Out Under Load
 

Hi Greg, I'm an apprentice owner so I'm not too versed on Z's yet but you mean that the car runs fine unless you lean on it?

Reading your description I was thinking how similar mine behaved when the injectors were starting to short out but mine would not run at all without me feeling & hearing & smelling the miss...load or no load.

You don't have a SES light, not even a winking SES light? You checked for codes? How are the plugs, wires, coils? Fuel pumps & regulator & filter are okay? Anything the ECM monitors or controls will yield a SES light if a malfunction happens. You don't have a scanner, like a Tech 1A, to scan the motor with? This is stupid but it didn't have a backfire? Those will blow off the MAP hose. Sorry I don't have something better to say like it's "this"! Do you have the Helm manual? Looks like you have to start at the beginning. The dealer or any shop would do the same steps in the absence of a SES light to give hints of where to start looking. You know spark? Fuel? Vacuum leaks?

ttt! One of the more knowledgable guys will see this & have something for you.
:thumbsup:
Tom

Re: 91 ZR-1 Cutting Out Under Load
 

No SES light, no blinking. Since it has never come on, I haven't checked for codes.

I don't have a Tech 1 but I do have an old Monitor 2000 which I have used before to find my secondary air flap solonoid not opening(showed rich on the right side bank via O2 sensor reading when WOT).

As far as I know, the plugs, wires, etc are fine. I'm not sure how to check that BOTH fuel pumps are working. That could be something to check into.

I do know that various vacuum lines can blow off from a backfire. I have had an occasional backfire upon startup. Most of my vacuum lines are zip tied on tighter because of this.

As far as the MAP hose, are both ends visable(to the map and to the plenum I'm assuming) ? or do you have to pull the plenum to see if the hose is connected?

Re: 91 ZR-1 Cutting Out Under Load
 

Both ends of the MAP hose are accessible without pulling the plenum.

Re: 91 ZR-1 Cutting Out Under Load
 

Greg, Like Tom said you can see the MAP hose at the rear of the plenum, it runs under the Fuel Regulator. There is a test procedure in the Helm for checking the pump out put pressure. I can type you a copy of the manual's procedure if you like. Oh, I don't have a Tech1A either. I have an old snap-on MT2000 with the proper GM software plug-in that I used to scan my motor.

There are two fuses, one on the regular panel and one on the stuff attached to the CDM box brkt. I'm doing this from memory so bear with me. The test is started with a KOEO test w/ a pres gauge attached at the schrader valve at the front of the right fuel rail. You can leave both fueses in & turn key on & see pressure. Should be about 50-55 psi. Then pull secondary pump fuse and repeat, should be 50 - 55 psi. Then pull primary fuse and repeat...50-55 psi. Then start motor both fueses in and pressure should be 45-50 psi, and shut down and see how fast it leaks down...I don't remember how much leak down is okay. The thing is the pumps only both run for the first little while...by the time the motor gets to "closed loop" w/no full power the secondary shuts off & remains off till the power key is turned on.

If you don't have a Helm I'll copy the exact info & post up....my recollection skills are not that good on something I don't do every day!:o

:thumbsup:
Tom

Re: 91 ZR-1 Cutting Out Under Load
 

Hey Greg sounds like you have the same problem that I had last year when one of my injectors was shorting out. The car would stumble and feel like the motor was coming apart if I tried to give it more gas. You can use a test light on each injector and it will tell you which one is not working good. My car worked fine until it warmed up and then it would start to act up. By all means check everything else out to be sure but I think that you will find that it is an injector especiall if they are original. I just replaced all of mine with RC 205's and my car is running better than it ever did, we put it on the dyno and it ran right out to the limiter with out missing a beat, good luck.
haber rj

Re: 91 ZR-1 Cutting Out Under Load
 

i guess you went through the easy stuff already..ie:fuel filter ,plug wires,etc?
blackjack

Re: 91 ZR-1 Cutting Out Under Load
 

Hey, it's not something simple like the air duct collapsing under load, is it? That thing seems to get softer with time. I recently bought a new one and it was much more rigid than my original one. A dirty air filter would also contribute to the pressure differential.

Re: 91 ZR-1 Cutting Out Under Load
 

Air duct collapsing just feels liek the secondary system cuts out. Just runs out of air and stops pulling.

Map sensor hose off will cause crappy idle and lots of black smoke coming out of the tail pipe.

Fuel pumps, to test secondary pump. Pull primary fuse and start the car. Car should run until temp comes up and enters closed loop. When it hits that point the car will die.


With the description from greg, primary injectors are fubar'd Replace and stuttering will go away.

Re: 91 ZR-1 Cutting Out Under Load
 

Ignition modules rarely go bad, but it sounds close to the same thing that my car as my ignition module was dieing. It was worse when hot. Has the plenum ever been off for porting or repainting (such that the dielectric/thermal grease may have been all wiped away)?

Good luck,

Marc

Re: 91 ZR-1 Cutting Out Under Load
 

Still trying to nail this cutting out under load problem down. This problem is something that just happened out of the blue, didn't slowly get worse. It also doesn't matter if it's bone cold or completely warmed up, same cutting out under load. Given that, does it still sound like an injector problem? I just don't see how one injector can make the whole car feel like it's falling on it's face.

One weird thing, this happed just after changing computer chips. I had been running a Rippie chip for years but it must have been a really old one because the car always idled way too high which was driving me crazy. I got the redesigned chip from GM that address that issue. I installed the GM chip the day the problem hit. Upon installing the GM chip, the car ran fine, idled correctly and I took my Sunday drive. It was later that day that the problem hit. Thinking that the chip might possibly be defective, I reinstalled the Rippie chip but the car still cut out just the same under load. It doesn't matter which chip is being used, same problem. Just thought I'd mention this. I can't image the chip change screwing something up but who knows? Anyone see any correlation?

Re: 91 ZR-1 Cutting Out Under Load
 

Just a coincidence.

Still sticking by my statement of primary injectors are going.

Re: 91 ZR-1 Cutting Out Under Load
 

Greg,

I tend to defer to Jeff.:worship: When my injectors went out to lunch it was just that quick. I don't actually know how many were bad as I never actually checked the injectors with an VOM. I used a scan tool to take the O2, block and integrator numbers. All that stuff was out of range & pointed to the injectors....that was after I checked the obvious: plugs, fuel filter, vacuum leaks, wires, fuel pump chart A7 diagnostic test...oh and every electrical connection I could reach also:o

The car was working fine...I shut it off & restarted and I had this slight miss, the more load the more miss...eventually it got so bad that I could smell a lean condition at the tail pipes and the plugs on one entire bank were not getting much fuel to ignite. Try a scan and post up the numbers and the guys that know that stuff will analyze the results and tell you what's up. They did that for me and taught me stuff that I had no clue about!:thumbsup:

:cheers:
Tom

Re: 91 ZR-1 Cutting Out Under Load
 

Thanks Tom. I will do that. As far as O2, block learn and integrator numbers, should they be at full throttle, idle, both?

Where should ZR-1 O2 numbers be at full throttle? I used to do a lot of that testing with LT-1's(should be 870-910) and TPI's (should be upper 700's to lower 800's). Wow, it's been years since I've hooked up the old Monitor 2000. I'm glad I still have it around.

Re: 91 ZR-1 Cutting Out Under Load
 

Greg,

The Helm FSM has a page in the "Emissions & Drivability" section that has all the function values that a healthy system presents to the ECM. The numbers are taken at idle speed, in "closed loop" operation. When I did my scan I followed the manual's instructions and just reported (posted-up) the values I was seeing here and the members explained what the data ment.

You'll have to forgive my lack of knowledge! The last vette I had before this one had a holley & the only electronic parts on the car were the ignition system and the radio!:o

:thumbsup:
Tom

Re: 91 ZR-1 Cutting Out Under Load
 

Tom, would it be possible to get a scan of that Helm's page with all the normal function values?

Thanks.
Greg

Re: 91 ZR-1 Cutting Out Under Load
 

Greg,

You got it man! I'll start to work on that now.

:thumbsup:
Tom

e-mail sent 8/15/06

Re: 91 ZR-1 Cutting Out Under Load
 

OK guys, finally got the time to test everything this evening. I hope someone out there can make something out of these numbers as I could find nothing wrong with the car.

1st. I did fuel pressure tests. Someone before mentioned that the FP should be around 50psi key on. On my car, key on I get 35psi. Car running, I get 30-32psi. These values are with the vacuum still connected to the fuel pressure regulator. I tried the tests with removing the primary and secondary fuel pump fuses individually. Each time, the car maintained 30-32 psi with the car running. When starting the car bone cold with the primary fuel pump fuse yanked, the car did just as it should which was to run just fine until warming up and then dying as the car went closed loop and the secondary FP shutting down just as it should.

I didn't have a real fuel pressure gauge, I was using the low side guage on my A/C gauges. I had another cheapy guage lying around and hooked it up too. With it I got the same 30-35 psi as my A/C gauge was giving. Are these pressure numbers OK? They did not seem to move below 30 psi when reving the engine.

With my Monitor 2000 and the car running at 180-195 degrees, I got the following values:

O2's - L- .10 to .69 R- .10 to .74
Block Learn L- 136 R- 130
Integrator L- 127 to130 R- 128 to 130
Inj Pulse Width 3.3 to 3.5
TPS .44
MAP 9 to 10


Anybody see anything wrong? I drove the car around and had the same problem, perfect idle, perfect cruise, stumble under load.

Re: 91 ZR-1 Cutting Out Under Load
 

Okay Greg I'm only an apprentice Z owner, since 12/04 & not out of my rookie time yet, but I'll take a stab & test what I might have learned.:o

The fuel pres #'s could be a little suspect as in a clogged fuel filter. The book gives way higher values. 55PSI @ idle on both pumps tested individually in the KOEO test. The idle speed fuel pressure should be no lower than 45 PSI or there abouts. Not to be forward but when was the last time the filter was changed? If the filter is done regularly then I would suspect the strainer socks in the tank might be clogged? The pressure regulator could be also the source of the consistant low fuel pressure at the fuel rail....I think??? I must go look that up & report back to you, sorry I don't remember but I'll check it.:o

The numbers from the scan look to be well within the "normal" limits defined by the FSM....TPS is a bit on the low side @ .44 but I doubt that is the source of the problem.....but I may not be correct on that statement and I'll try to look that up also.

Sorry I'm not very definitive but I'm not too secure in my abilities at my experience level.:o

:thumbsup:
Tom

Re: 91 ZR-1 Cutting Out Under Load
 

Thanks Tom. I think tonight I will get the car out of the garage, set the Monitor 2000 on record mode and see what kind of O2 numbers I get at full throttle. If they are both lean, than it's time to change the fuel filter and check out the strainer. I can't imagine how the fuel pressure regulator could cause the pressure to drop consistantly.

Actually, I did drive the car around the block last night and while it did cut out, it didn't seem as bad as it was before.

Anybody know what the O2 sensor readings should be at full throttle? I'm thinking in the .800's but not sure.

Re: 91 ZR-1 Cutting Out Under Load
 

Greg,
Let me run this by you about the O2 sensor #'s. The sensors should respond to changes in the A/F ratio as adjusted by the ECM, therefore there will be no set number in mV's that appears. Near as I can figure if the mV's are not constantly varying then the sensors are getting lazy and or going out to lunch. They react or read what has already been done by the ECM to the A/F ratio via the injector pulse width changes. In short I think they either work or don't with a lazy phase prior to quiting all together.

I still don't like the low fuel PSI readings.....using the A/C gauge set not withstanding....book says 48 - 55 PSI in the KOEO test for both 1* & 2* pumps, and a 3 - 10 lb drop with the motor at idle speed as within specs. The f/f is an easy swap & might yield something, it's also the cheapest part in the fuel system!:) I know I tested my fuel PSI when I was having injector trouble and the difference between a new filter and the one that was in the car for 8K miles was dramatic, oh yea the OEM filter was still in my car when it was delivered.....a 15 yr old f/f is not my idea of good maintenance. I do it every yr along with some other stuff.

The regulator seems to either work or not, according to the FSM. Oh, the only variable is it's vacuum source....it is good or the hose is at fault. Check to see the hose is okay and that there is no fuel in the hose. Fuel in the hose is the tell tail that the regulator is kaput.

:cheers:
Tom

Re: 91 ZR-1 Cutting Out Under Load
 

I hear ya' Tom. I wouldn't be surprised if my car still has the original OEM filter. I'll definitely change that and see what it does to my pressure.

As for O2 reading, yes they change rapidly and dramatically at idle and at cruise speed. At full throttle, they go up way high and should be somewhere around .750-.900 and stay in a certain range. That's how we tell how to adjust our adjustable fuel pressure regulators at the drag strip. On tuned port injection cars(I still have one with a Lingenfelter 383) the full throttle O2 readings should be around .760-.790. On LT-1 cars, optimal readings are around .860-.910. I just can't remember what ZR-1's are supposed to be.

Using these full throttle readings is how I discovered once on my ZR-1 that a little 5 cent clip had fallen off the right side secondary flap actuator. Without that actuator opening the secondary air flaps, the O2 senser readings on the right side went wacky.

Re: 91 ZR-1 Cutting Out Under Load
 

GM uses short-term(integrator) and long-term(block-learn) fuel numbers. They are as they say, fuel maps of different time lengths. The short-term numbers will be very bumpy and jump all over the place, even at idle. This short-term map varies fuel for any reason such as a lead foot, hilly route, large load, a/c,vacuum leak, etc. It instantly adjusts with (in GM's case) above the number 128(the mean desired fuel delivery point) representing lean or adding fuel, and below 128 as rich and subtracting fuel. The block learn or long term numbers are a smoothed out picture of this fuel delivery. The same 128 number representing the optimum mean point. Above 128 and the PCM is needing to add fuel over the long term and vice-versa.

O2's - L- .10 to .69 R- .10 to .74 What is the rate of fluctuation? Slow/Fast?
Block Learn L- 136 R- 130 You are adding fuel, not that much though
Integrator L- 127 to130 R- 128 to 130
TPS .44 At idle or key in the run position should be around .54 volts


When testing injectors, remember that checking the resistence is only 1 of the things that can fail. You can have it also fail by leaking, spray and flow issues.

Consider spending the ten bucks on a fuel filter, make sure you have line wrenches for the job.

Re: 91 ZR-1 Cutting Out Under Load
 

O2's at idle are fluctuating as normal, very quickly.

Trying to remember on the TPS, is the ZR1 TPS non-adjustable or the same as the TPI cars at the time that should be adjusted to .54 +/- .05 ?

Re: 91 ZR-1 Cutting Out Under Load
 

It is adjustable. Takes a T-25 to undo the screws. It's auto correcting within certain limits. And .44 is not with those limits.

Re: 91 ZR-1 Cutting Out Under Load
 

Finally got a chance to work on the beast again. I pulled the fuel filter out and it had come apart, it rattled. That gave me some hope but I could still easily blow thru it. I put the new filter on and still had the same fuel pressure reading. Went out for a drive, same problem, stutters under acceleration. I recorded my O2 sensor readings and they appear to be just fine. Under full throttle acceleration the O2 readings are:

Left: .84 - .87
Right: .85 - .88

I'm now officially stumped. With the fuel filter coming apart, I wonder if a bunch of crap got into the injectors ??? When I undid the fuel lines and pulled them away from the original filter, I put a bowl under the lines to catch the fuel. The bowl got a bunch of dark particles in it. I wonder if some of that stuff made it into the injectors??? You would think however if the injectors were clogged up, the O2 readings at full throttle would go lean while stumbling.

I don't know, officially stumped now.

Re: 91 ZR-1 Cutting Out Under Load
 

Yikes, hate chasing problems like that, hope it gets solved soon.


Jeff, I take it the setting on the TPS is supposed to be .54?
Sorry bout the hijack.

Re: 91 ZR-1 Cutting Out Under Load
 

As far as the TPS, they are not adjustable. Mine is .44V at idle and 4.4V at full throttle. You can loosen the 2 screws holding the TPS and move it just a tiny, tiny amount that doesn't even affect the numbers. This TPS does not have the adjustable slots like on earlier TPI motors.

Re: 91 ZR-1 Cutting Out Under Load
 

Ok, I got confused...thought they had screws for adustment, but now its clear they arent. Good news actually, one less thing to worry about

Re: 91 ZR-1 Cutting Out Under Load
 

It is adjustable, it swings up and down after loosening the two screws that hold it on. The acceptable range is .46 to .62 volts at closed throttle.

Re: 91 ZR-1 Cutting Out Under Load
 

Mine doesn't. It has 2 fixed screw locations with no pivot, no slots, no adjustment.

Re: 91 ZR-1 Cutting Out Under Load
 

you need to take off the cover

Re: 91 ZR-1 Cutting Out Under Load
 

Take off the cover on what? There's nothing to take apart on the TPS. It's a sealed unit just like every other TPS GM has been using since about 1990.

Re: 91 ZR-1 Cutting Out Under Load
 

I was refering to the silver cover that covers the tps partialily and is conected to the throttle body it coveres the torx screws

Re: 91 ZR-1 Cutting Out Under Load
 

This is what it should look like. There are slots where the screw goes through. Plenty of room for adjustment

http://www.pnwzr1.net/Jeff/TPS.jpg

Re: 91 ZR-1 Cutting Out Under Load
 

Oh man, am I stupid. I was looking at the passenger side of the throttle body for the TPS, just like a TPI or LT-1 car. I guess that's the IAC motor is there. You're right, the TPS is just as you said but on the drivers side.

Re: 91 ZR-1 Cutting Out Under Load
 

Finally got some time to work on the car today and yanked the upper plenum and replaced the plugs and wires. This was at the advice of 2 Mercury Marine ZR-1 guys who know the cars. Car fired up perfectly afterwords but as soon as it warmed up a little........... same damn problem!!!! misfire under load. At this point I'm guessing primary injectors or computer. I'm really getting tired of this.

The car has been down for almost 4 months now and I really don't have the time to keep chasing it. Somebody convince me to find to the problem and not sell this car. At this point I'm really hating the car. I've owned it for 7 years, have put on about 1500 miles and have had to take the plenum off 4 times for various problems.

Re: 91 ZR-1 Cutting Out Under Load
 

Greg,

I just want to say that your experience is not isolated. I went thru the same thing when my 1* injectors went out to lunch. The car had the same "miss" but not just under load. You could hear & feel it at idle. I learned one thing thru all the trials I've had with mine since I became a Z owner.....learn how to work on the car myself. That, and the first owner knew when to say when...he sold it before it drove him nuts.

I also hate to say this but I'm leaning to the alky getting to the injector coils and the "junk" in the F/F not helping the spray pattern on the nozzles. The only trouble is RC's are 1100 for the set...or there abouts. The GM stuff I think is more and then there are the Accels recently discovered that are like half the price of either of the previously named parts. The skinny on the accels is you need to call Mr. Haibeck & get different O-rings for the primaries to work. I did mine last year about this time and used the 205 RC, $1064.00 delivered for all 16. Registry members get a discount down to 68 ea.

Ofcourse when I pulled the plenum I changed a bunch of stuff besides the injectors. CCV box was leaking so gasket & hoses, 2* solenoid was N/G, coils & wires, vacuum hose assembly, tank, both temp sensors, 2* actuators, cleaned up the harness grounds, di-electric on all the weather tight connector gaskets, etc, etc...I was paranoid!:sign10:

I was starting to regret, or question the wisdom of my purchase...truth! The only thing is when it was done & the baby fires up....man it was all:mrgreen: again. The cash put a dent in the budget for sure but I did by a used car after all. Oh, and the punch list ain't done yet....I'm still playing catch up, and it isn't even anything fun like a P & P job or headers or anything to get me closer to the C5's & C6's I hang with on cruises! I'm one of the only antiques in our little cruise group...but I keep up!:D

I guess what I'm saying is hang in there & when you solve the problem you will feel good about what you've learned! Then when you go for a ride you'll even feel better....I know I do!
:thumbsup:
Tom

Re: 91 ZR-1 Cutting Out Under Load
 

I'm curious why you haven't sent the original injectors off for disassemble, cleaning and rebuild ? It's been many years since I've done that but when I did, it made a world of difference in TPI cars from the same time period. When they do that, they check the spray pattern and make sure the injectors are working properly.

Sounds like the easiest and least expensive thing to try now might be to order a new computer from GM. It would sure help if the car would throw a check engine light.

Re: 91 ZR-1 Cutting Out Under Load
 

I hear your frustration and know you do not want to bite the bullet and go the injector route, but sorry to say that is most likely your problem.

As to not selling I can give you several reasons but here are two very good ones . First is that when your baby is running right you have a great time that cannot be matched by other cars, especially in a moderate price range (assumption on $$ :wink: ). The second reason is that if you were to try and sell it with the existing problem, you are not going to get as much for it as you would if it were in tip top shape. The bottom line is that you bought it for a reason and just need to get it running right to recapture the great feeling of owning a KOTH

:cheers: