Engine is missing

[mrzzzr1]

My LT5 moter started missing badly recently.

Sometimes only after warm up.

Other times right from a cold start. When missing engine lopes badly at idle and makes a knock sound but only at idle. My mechanic has some LT5 motor experience and thinks the knock at idle is the dual mass flywheel making noise due to the way the engine lopes due to missing. It only makes this sound when it is missing/running badly.

Sometimes it is running smoothly with no apparent problem.

Oil pressure drops near zero sometimes at idle but reads near 80 on the highway. We originally thought it might just be a bad oil sending unit but now I am starting to get worried.

Any thoughts or suggestions would be appreciated.

[tomtom72]

Have your mechanic put the scaner on the car and look at the fuel numbers. You know the Block Learn #' and the Integrator #'s and the O2 sensor cross counts. Those will let you "see" what the state of the fuel trim is. The 90's have the injectors that will eventually fail, via shorting coils, due to the modern alcohol fuel additives. The ethanol in gas now was not an additive when the injectors were designed, therefore the coil insulation is not resistant to it.

You are sure the coils, plugs & plug wires are okay? There is no SES light indicating any stored DTC's? You're sure the fuel filter is not blocked? Those are some of the simple things to "eye ball".

The behavior you describe sure sounds like the primary injectors are shorting out. When they short out they do not shoot any fuel and you wind up with a big miss or skip. That miss causes the D/M fly wheel to "Knock" because of the uneven power pulses from the cylinders. If you stand behind the car does it smell "Lean"? That's another give away on a fuel issue. If you pull the plugs you might see which are not getting any gas...they look to clean.

The symptoms you describe are almost exactly like what happens when the injectors start to short. Usually there is a slight miss that gets worse as the motor come up to normal operating temp on the coolant. The coolant temp is not affecting the injectors. As the injector coils are worked more they get hotter and if they have shorts the short becomes more noticable, you get a bigger miss. OH, forgot this: you may want to verify that you have the correct fuel pressure also. You do have a FSM from Helm for your model yr?


Tom

[tccrab]

I agree with Tom, first thing to check is the primary fuel injectors, then replace the oil pressure sending unit.
Fuel injector failure is fairly common in the early year cars, it's caused by ethanol in today's gasoline that eats through internal seals in the fuel injector and shorting out the coils. There is no "Fix" only replacement.
The oil pressure sending unit is another common failure item on early year cars.

Tom and Tom are hitting in the ballpark. I would like to ask, this appears to have been a issue that has recently appeared. What has you use of the car been in the previous 4 months. Driving the car? Letting it sit? Limited use?


Things to check
Air fuel spark. More than likely air is not the issue, so it boils down to fuel and spark. Check the plugs for any indication of a lean miss or fouling of the plugs. This should always be your first step and then go from there.

[guinnessdood]

You could have a couple of problems. The oil pressure indication problem may indeed be the sensor (sending unit). You can get a replacement sensor at most auto parts stores and changing it is a snap. If you remove the electrical connector from the sending unit and there is oil in the connector, then it is definitely bad...what happens is the oil leaks through the sending unit from the oil side to the electrical side and causes erratic readings on the oil pressure gauge.

The other issue with the rough running should be troubleshot as described already...

[mrzzzr1]

You are right on. The injectors turned up bad.

[mrzzzr1]

It has sat during the winter months. It appears that contributes to the problem. PS- my mechanich found out it was the injectors.
Thanks.

[tomtom72]

Glad you guys found the problem. Yes the early cars 90-91 seem to be the most susceptable to lack of use where the injectors are concerned. Remember that if you go to OEM?GM injectors that GM never "corrected" the seals for ethanol.

I think the Delphi injectors are corrected. I also know that a bunch of people have used the Accel injectors with very good results. There are threads about which P/N Accel's to use here. And there are always the RCEngineering.com injectors we get a registry discount too from them.

JMHO on this job if I may. Don't piece this job because they will all fail the same way. Just bite the bullet and do them all(16) at once so you only have to do it once. It's a lot of cash but in the long run it's just better to be done with it.

You also might want to consider giving the secondary port throttle system a very critical "eye ball" while you're under there. Ya know the control solenoid, tank, vacuum line spyder, secondary actuators, check the CCVBox cover for loose bolts & leaking gasket & hoses, clean the valley & starter, load test the alternator, plug wires and coils, A/C system vacuum control valve, temp sensors(in the left I/H, up forward), serp belt, make sure the screws on the secondary butterflys are tight even a drop of lock tite on them is cool(one guy had one fall off & go into a combustion chamber). Or maybe even a port & polish on the plenum, I/H's, T/body with some new powder coat!

Sorry for spending your $. Point is the gaskets are not cheap and GM don't have 1/2 of them anyway. GM don't sell them in a set either, just all seperate. Try partszoneonline.com (P & G chevy in PA) or Superrior Chevy or Marc Haibeck (you could ask him the best source for the gaskets).

Okay I'll shut up now!


Tom

[Norsky4360]

This thread (and the others related to this topic) have been helpful to me. "The Beast" has developed a miss (and the knock at idle) within 300 miles of purchase. 200 of those miles were put on late last fall before parking it for the winter. With a 4k road trip coming up in less than a month I'm just going to bite the bullet and replace all of the injectors as soon as possible. With just under 8k on the odometer the originals have had a lot of time to go south from non-use.

Thanks!

[tomtom72]

Hey Jim you're welcome!

That's why all of us are members of the registry....together we can solve most of what goes wrong and we all share the light!

Now it's time for a few things I forgot!
Good write-ups on this are at Pacific Northwest Chapter site and at Corvette Action Center.com in the C4 section under Tech and performance/ZR-1. They both have orientation info that is worth reading about doing the plenum pull(big thanks to Jeffvette @ PNW ) and secondary vacuum system from Rob over at CAC.com. Check the stuff out guys it has a wealth of info! Also over at the NetReg site in the How To section there is a bunch of info also.

Tom