92 Z NIGHTMARE

[Vettetech1]

Ok so I started this in the knock sensor holding you back thread and decided to start a new one. After replacing some injectors the car ran great. It sat a few weeks and the customer picked it up and said it was running worse then when he brought it to us. So after the weekend i came back in to see this black beast staring at me. I go and drive it and sure as hell its running like it did before i did the injectors. So i pull the plenum per my boss and recheck the injectors all ohm good none leaking all spraying. Put the plenum back on it and go to start it, Nothing. Found the starter had crapped out. So off the plenum comes again 3rd time in 3 weeks, get down to the starter and low and behold, a mouse created a home. Plugged the drain chewed on the purple starter wire and managed to kill the starter with his lil rat bladder. Got the starter in put the car back together (which im getting pretty good at if i dont say so myself). Hit the key it fires up, running nice and smooth, sweet thought i fixed it. NO (car god smacks me in the face). Pretty sure I got it down to a ignition miss, anybody have any tricks to finding a miss? Pulled plugs all look good like its running great. Regapped and reinstalled but the miss is still there. If anybody has any suggestions or ideas I'm all ears.
Car is a 92 with 24000 on the clock OEM AC Delco plugs, driven not beaten.

[Schrade]

No codes? No Closed Loop codes?

Short of that, can you post a datalog scan?

[Jim Nolan]

If I saw a mouse had made a home under the plenum I would have checked all the wiring and the plug wires. Those buggers are noted for chewing through wiring shields to make their nest. It maybe grounding out from a bare wire. 24k tells me it sits a lot even if an garage the mice can get to it, personal experience.

[Vettetech1]

No codes, seems to be going through o2 loops like it should. Yeah checked all wiring didn't find anything but the starter wire chewed on. Found moister in cylinder 8 plug well dried it up, seemed to take care of the miss. Now its got a hesitation sag when you jump on it. both in neutral and when driving down the road. If you ease through the throttle seems to be OK. Something to ponder through the weekend its almost 4 and 100+ in the shop i am out for the day. Thanks for your thoughts and suggestions not real familiar with the LT5's.

[XfireZ51]

Did you say you checked the fuel system? Pumps, strainers, fuel filter by passenger front?

[LGAFF]

2ndry map sensor under the ECM?

verify secondaries are opening, you can see them inside the plenum.....

[Schrade]

No codes, seems to be going through o2 loops like it should. Yeah checked all wiring didn't find anything but the starter wire chewed on. Found moister in cylinder 8 plug well dried it up, seemed to take care of the miss. Now its got a hesitation sag when you jump on it. both in neutral and when driving down the road. If you ease through the throttle seems to be OK. Something to ponder through the weekend its almost 4 and 100+ in the shop i am out for the day. Thanks for your thoughts and suggestions not real familiar with the LT5's.

I think that idle quality and idle parameters will tell more about the tune, than road /engine response...

Could be wrong tho'.

[rhipsher]

This is what I'd do if my Z started to act up like the above described. I'd force it to throw an SES which in turn will store a code. If its stumbling when you get on it than you need to really get on it. I can almost assure you that when you hit somewhere around 7k the SES light will appear in the information center and will turn off when you back off of the throttle. And there's your code. I was also going to suggest checking to see if your MAP sensor hose is plugged in. It sounds like a stupid thing to overlook but I think we've all done it at one time or another. That will cause terrible idle and run like crap. Once it's plugged back in it will purr like a kitten. But the code will tell you where to look.

[Blue Flame Restorations]

I had an intermittemt miss that turned out to be a weak fuel pump. replaced both pumps and problem went away.

You need codes to appear

[Vettetech1]

I had a knock code throw at one point. That's what brought me to this forum. But I haven't been able to get it to come back. The vacuum hose to the map sensor on the back of the plenum is new. The only worry spot I just thought about is the elbow that connects the two Pcv valves to the vacuum tube. Come to think about it they seemed pretty sloppy, it's got hose clamps on the bottom side. Any way to seal the top better without buying a new elbow. That could be part of the off idle stumble/hesitation. Been a long hot week maybe next week on a fresh brain will help. I'm sure it's something dumb.

[Vettetech1]

I had an intermittemt miss that turned out to be a weak fuel pump. replaced both pumps and problem went away.

You need codes to appear

CODES would be great!!!! I'm the tech for ACI and most things that go wrong on corvettes I know from experience. I haven t had the pleasure of too many LT5 cars however. (Would like to get my hands on one to stuff in my 93z28. Hmmm fun times. )I'm starting to figure out stuff tho. I thought about the fuel pumps too, maybe Monday I will recheck fuel pressures.

[rhipsher]

You definitely came to the right place for help with this unique Corvette. This crew makes it their personal mission to help troubleshoot and fix any problems with the LT5. The wealth of technical and mechanical knowledge is huge here.

[Paul Workman]

Some things that caused my Z to fall on its nose...

FUEL...

Secondary fuel pump going south...

With pressure gauge hooked to the Schrader valve, and rolling the ignition switch on w/o starting the engine, both pumps should kick on for a second or two, and the pressure gauge may peak at high 40s or even above 50# before settling at mid 40s. Pressure should hold to a pound or so of that for (some say) 30 minutes. (A brand new secondary pump had a bad check valve on the first one I replaced. Take nothing for granted).

Taping a pressure gauge to the windshield put the finger on another problem I had which turned out to be a filter that was clogged.

http://i185.photobucket.com/albums/x220/6PPC_bucket/tech%20files/FPtestLarge.jpg (http://s185.photobucket.com/user/6PPC_bucket/media/tech%20files/FPtestLarge.jpg.html)

Pressure should be: Mid 40s at cruise, 52#± at WOT - all the way to 7000+

You can also activate both fuel pumps and by measuring the current draw, determine if one of the pumps (or both) are not working, again coupled with a fuel pressure gauge attached to the Schrader.

Inboard of the diagnostic connector, near the brake reservoirs, there is a red wire poking out of the wire loom about an inch with a connector on it. With the ignition switch OFF, taking a current reading between the pos post of the battery and that connector (through a VOM set up to read 10A), you should read between 8 to 10A for both pumps. A reading half that indicates one or the other is not working.

http://i185.photobucket.com/albums/x220/6PPC_bucket/tech%20files/Fuelpumpcurrentcheck001.jpg (http://s185.photobucket.com/user/6PPC_bucket/media/tech%20files/Fuelpumpcurrentcheck001.jpg.html)

Note the alligator clip lead connecting the red test connector to the BLACK lead of the meter. The Red lead goes to the POS on the battery

In this case, both pumps are working normally.

http://i185.photobucket.com/albums/x220/6PPC_bucket/tech%20files/Fuelpumpcurrentcheck005.jpg (http://s185.photobucket.com/user/6PPC_bucket/media/tech%20files/Fuelpumpcurrentcheck005.jpg.html)

The resistance of the injectors should be checked when the engine is at normal operating temperatures. Quite often an electrical coil will read normally at room temp, but the failure doesn't manifest itself until heat is in the mix.

If there is an easier way to measure injector resistance besides accessing the relay batch connector, I'm all ears. But, in reality, it is pretty easy to do, and if you don't have a factory service manual (see if the owner has one), I or someone here can dig up the pin-out procedure for measuring the injector resistance w/o pulling the plenum or unplugging the injectors.

Anyways, injectors that are OK will read within a range of a couple ohms or less, across all 16. Injectors reading 10 ohms are suspect, and if they go as far as 7-8 ohms or less, you can expect trouble with idle and smooth operation on the one hand, and poor performance or even burning a valve (worst case...Ask me how I know!).

AIR...

Falling on its nose at WOT, around 4500-55000 or so rpm might be the plastic accordion tube collapsing that is connecting the air horn with the air filter box. The fix is as simple as wire hoops placed between the pleats of the tube - 4 spread across the middle should do it - or fashioning an aluminum sleeve that fits snuggly inside the tube (my favorite).

It IAC valve on the passenger side of the throttle body needs to be checked for carbon and be cleaned as needed.

Idle that remains high (1800-2200, for example) after 5 seconds or so is usually due to air leaking past a gasket or that IAC valve or a hose e.g., the brake booster not connected.

The vacuum pump typically runs for under 5 seconds when turning the switch to ON, and then shuts off. If it cycles on then off after about 10 seconds, there is a leak, but not serious enough to impair the SPT operation, due to lack of vacuum.

Far as the Secondary Port Throttles go, Marc Haibeck's (http://zr1specialist.com/)web site has a tech article describing how to check them without pulling the plenum.

Note: The fact that the SPTs can be eliminated with minimal negative affect on the total operation, makes removing them an option, especially if parts are not available, or as part of a porting project. However, a chip that is modified to operate all 16 injectors full time is required, and you NEVER want to switch the POWER switch away from the FULL POWER setting...extremely important! (I shunted my (90) switch against accidents or internal switch connection issues.)


SPARK...

Spark issues, as you are well aware, can be wires, plugs, or coils...or in the case of the LT5, the ECM or DIC. But, first things first:

The LT5 uses a waste spark ignition, i.e., each coil fires a pair of plugs each, at the same time. Tho you can pull a boot off of each plug while the engine runs to find a cylinder(s) not firing, beware of mouse-chewed plug wires in the vicinity of the coils!! Marc Haibeck (one of the ZR-1 specialists in the Chicago area) warns that the wire harness to the ECM runs back there too, and if a mouse nipped a pin-hole in one of the plug wires, it might be impossible to see, but could zap the ECM harness, and then you'd have a BIG problem. I've checked my plugs this way, I admit, but I also had new plug wires (after mice chewed my original wires).

Coils too exhibit failure when hot, yet when comparing resistance of the primary and secondary sides of the coils when cool, they may all be more or less equal. Failure only when hot is a classic coil/winding phenomenon, in my experience, beit windings of coils or of motors!

Note: Replacing an item does not eliminate it as being at fault. Brand new chit is bad out of the box. The trap people get into is assuming because a faulty piece was replaced that that device can be eliminated from suspicion. BULL PUCKY! Never assume! Nothing is known but what we verify...is a golden (troubleshooting) rule.

That said, and a coil (for example) is replaced, yet the problem still remains with that cylinder(s), and swapping coils with one known to be working does not fix the problem, then just for peace of mind, I'd want to verify that none of the injector wires got pinched (happens often enough), and a compression check does not indicate anything drastic (e.g., valve issue, for example). If the problem still exists, then swapping the ECM with a known good one would be in order.

Oh, and one other thing that bites people is bending one of the many pins on that large connector going to the DIC that has the snugging bolt in the middle of it. Much care needs to be exercised when reconnecting that connector especially. (also goes for ANY connector, but those connected to the ECM too are especially touchy.) Also, while I'm on it, you never want to jam a test probe into the female sockets in the ECM (or any) connector, as they are easily damaged to where they won't grip the pins afterward.

Summarizing...

This is rather cursory, I admit. More info will narrow the focus (for all of us). Something should come out of this, I would think. But, the one thing I've not seen mentioned here is executing a data logging test. That way real-time sensor data can be analyzed and plotted that will show stuff even before a code is set; especially if the problem is intermittent or of short duration. Do you have a laptop with data logging capabilities??

Hope this helps.

P.

[A26B]

.........The only worry spot I just thought about is the elbow that connects the two Pcv valves to the vacuum tube. Come to think about it they seemed pretty sloppy, it's got hose clamps on the bottom side. Any way to seal the top better without buying a new elbow. That could be part of the off idle stumble/hesitation. .............

I have new top connectors for the PCV valves in stock.

http://jerrysgaskets.com/store2/root/index.php?main_page=product_info&cPath=7&products_id=76

Fixing vacuum leaks is good, but I don't think this is your problem.

I have seen weak spark several times, as the cause of an intermittent miss. In all of those instances, a coil was at fault.

Use an extra plug and check spark to ground on each cylinder with the engine running at idle. Spark should be bright blue & jump 1/2"~3/4". If you have an orange spark, replace the coil.

I would also recommend replacing the plug wires at the same time and use dielectric white silicone grease on the coil terminals and in the boots on each end of the plug wires.

http://jerrysgaskets.com/store2/root/index.php?main_page=product_info&cPath=22&products_id=547

Considering you had a nest under the plenum, clean each electrical connector, lube the contacts with the silicone grease and also the connector seals.

[Vettetech1]

Got the miss figured out it was moister in number 8 plug well. Now i got a hesitation off idle. Fuel pressure is good 55- 60 WOT no sag in fuel pressure coming off idle. I don't have data logging software.

[XfireZ51]

What did u ever do about the PCV slop? Have you checked TPS operation and Closed throttle voltage? Min Air settings?

[Franke]

Vettetech1, throttle position sensors are famous for off idle sag. They can be tested with a ohm meter to see if there is a dip in the resistance just as the sensor starts to move off idle. Its a potentiometer and varies the voltage to the ecm.

[Vettetech1]

Vettetech1, throttle position sensors are famous for off idle sag. They can be tested with a ohm meter to see if there is a dip in the resistance just as the sensor starts to move off idle. Its a potentiometer and varies the voltage to the ecm.

Watched it on my Solus but don't see a dip. I will have to check with an ohm meter. Its odd it only does is when you stuff it. If you gradually ease through the throttle it is fine.

Also I know there is a MAP sensor on the back of the Intake. Is there another sensor for Vacuum on these cars.

My Solus shows 37KPA at idle on the MAP then there is reading for vacuum witch is 0 with .58 volts.

My IAC is at 16 counts at 600 rpm does that sound normal?

[Vettetech1]

What did u ever do about the PCV slop? Have you checked TPS operation and Closed throttle voltage? Min Air settings?

Was able to use head shrink on the nipples of the PCV valves to tighten them up in the elbow. Closed throttle TPS is .52 volts. smooth counts through the range stopping at 3.9 WOT

[XfireZ51]

Was able to use head shrink on the nipples of the PCV valves to tighten them up in the elbow. Closed throttle TPS is .52 volts. smooth counts through the range stopping at 3.9 WOT

TPS v is slightly low. Try setting it to .55v although WOT voltage should be in the 4.3-4.5v range. IAC counts look good at idle. 37kPa is pretty good vacuum at idle, almost sounds too high but
I don't understand your comment on vacuum being 0 w .58v.
When you take it through the throttle, can u get beyond 3k rpm?

[Vettetech1]

TPS v is slightly low. Try setting it to .55v although WOT voltage should be in the 4.3-4.5v range. IAC counts look good at idle. 37kPa is pretty good vacuum at idle, almost sounds too high but
I don't understand your comment on vacuum being 0 w .58v.
When you take it through the throttle, can u get beyond 3k rpm?

I don't get the Vacuum reading either maybe just an extra parameter that me scanner has for a sensor the car doesn't have. Ill try bumping the TPS up some and see if that helps It will run up till 7k. Just stumbles/hesitates when first getting on it and happens with every gear shift.

[Vettetech1]

No luck after bumping the tps up to .56

[LGAFF]

Did you check to make sure the secondary map sensor under the ECM is hooked up?????

[Franke]

The FSM shows that the sensor under the ECM is the secondary port throttle vacuum sensor. It monitors how much vacuum is applied to the secondary actuators. I don't think that would be an issue here unless it is a source of a vacuum leak. A failure of this sensor should set a code 56.

[Franke]

Vettetech1, Have all the fuel injectors been replaced? Especially the primaries. As Paul says above, they need to be checked while hot. They will read between 12 and 13 ohms cold but lower than that when hot. My car had 4 bad injectors on the secondaries and 1 on the primaries. My symptoms were.. stable idle in open loop then miss fire and sag when in closed loop. Of course closed loop is when the engine temp is up and the O2's are providing feedback to the ECM. So when the injectors started to fail I could see this using TTS datamaster and laptop as the fuel trim went nuts. Something else I had was excessive clutch noise in neutral due to the slight missfire. That all cleared up when I replaced all 16 injectors.

[Vettetech1]

Vettetech1, Have all the fuel injectors been replaced? Especially the primaries. As Paul says above, they need to be checked while hot. They will read between 12 and 13 ohms cold but lower than that when hot. My car had 4 bad injectors on the secondaries and 1 on the primaries. My symptoms were.. stable idle in open loop then miss fire and sag when in closed loop. Of course closed loop is when the engine temp is up and the O2's are providing feedback to the ECM. So when the injectors started to fail I could see this using TTS datamaster and laptop as the fuel trim went nuts. Something else I had was excessive clutch noise in neutral due to the slight missfire. That all cleared up when I replaced all 16 injectors.

Over the past 3 years all 16 injectors have been replaced. It came in a few weeks ago running like crap after it was warmed up in power mode. Found 2 bad secondary injectors. After looking at his repair history i noticed there were only 2 more injectors after that were OE. So i suggested we do the last 4 injectors so we wouldn't go through this next year. After doing the injectors it ran great on the test drive. No hesitation or anything. Owner picked up the car a few weeks later and boom running like crap again. Tested injectors both hot and cold are around 12 ohms.

[Karl]

Were the injectors new or rebuilt?

To my understanding rebuild will.bb fail again eventually.

[Schrade]

deleted for new thread

[Vettetech1]

Well just got a chance to take it out on the road. Even tho there is a stumble sitting in neutral, you cant notice it while driving it on the road. Maybe I'm just being to critical of it in the bay.

[Vettetech1]

Were the injectors new or rebuilt?

To my understanding rebuild will.bb fail again eventually.

they were new

[XfireZ51]

Not sure what you mean by stumble. A stock LT-5 will exhibit a moment of hesitation when transitioning between Port Throttle Closed to Port Throttle Open. Secondary Injectors don't go on immediately as you open port throttles. They wait for the throttle blades to be opened. its in the calibration. Is that what you are seeing?