91 ZR-1 Cutting Out Under Load

[GregCrowe]

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.

[tomtom72]

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.

Tom

[GregCrowe]

Originally Posted by tomtom72

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.

Tom

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?

[Tom]

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

[tomtom72]

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!


Tom

[haber rj]

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

[Aurora40]

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.

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.

[mrand]

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

[GregCrowe]

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?