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Hi All, I'm dealing with a 1990 Corvette that has a low rpm misfire. the car has 50k miles on it and I thought nothing of replacing the tune up parts that were all original to cure this issue. Only problem was it didn't resolve the problem. I started digging further and found the EGR valve wasn't working due to a bad diaphragm. I didn't figure this was a big deal but used it as a reason to get to the injectors for inspection. The screens on all of the injectors has very fine rust particles in them but they all ohm tested the same at 16ohms. I cleaned the screens and reinstalled the injectors, also replacing the egr valve. No difference. I did a coil cancelling cylinder balancing test to try to figure out the weak cylinder(s) and it was very unclear as to the offender(s). The customer insisted I replace the coil and module so I ordered a a complete distributor from Rock auto. Still no change. A local garage told me that the Multec injectors are garbage (even though I balance tested them, thanks to Scanner Danner for that tutorial) and that I should replace those. I bought a set of 22lb injectors from FIC and installed those. No improvement. I must say, this is as much trouble as I've ever had diagnosing this type of "old school" technology. Other tests include a vacuum test of the crankcase looking for a bad intake gasket, watching the O2 BLM, (which reads 144 at idle, a little lean), and integrator which holds pretty well at 128. Compression test and all cylinders are above 150PSIG. My MAP reading is 20" at idle. This is an SD engine so it doesn't have a MAF sensor. What am I overlooking here?
This is tuned port right? Yes BLM is way lean. Is a vacuum gauge stable at idle? Any backfiring? Exhaust leaks before the o2 will absolutely give lean engine issues. Relative compression test look good? What's your fuel press and pump amperage? The reluctor on the distributor mainshaft isn't loose? Did someone set the timing without putting into bypass mode? Is the vacuum source to the map sensor open and clean? I had one that the port was almost completely clogged and it caused all kinds of strange issues.
Yes this is a port injection system. Injectors are bank fired. I have not checked fuel pump amperage. Pressure is 38 at idle and 45 on throttle snap or vac hose off. I only did a cranking compression test. I set the timing with the spout unplugged when I replaced the distributor. There are no exhaust leaks on either manifold. I believe it only has one O2 sensor. I will check the vacuum line to the map sensor but reading it on my OTC seems to be accurate. I will put a vacuum gauge on the manifold and see how it looks. Thanks for the reply and suggestions.
Do you think it is a secondary type misfire or a lean idle misfire? Unless the vac gauge shows something I would say not mechanical. When is the misfire? I'm assuming you've already replaced the wearable tune up parts?
Does it feel like a single cylinder misfire and is it a constant miss? Failing at low rpm may indicate a sealing issue so the vacuum gauge is a very good next move. If so equipped scope the 2.ary ignition and the intake vacuum; if lacking a pulse sensor take advantage of the MAP.
An expert is someone who knows each time more on each time less, until he finally knows absolutely everything about absolutely nothing.
I'm guessing lean misfire considering I've replaced all tune up components and the distributor assembly. I have a hard time believing it could be the pcm considering it runs so strong above 1500 rpm.
To me I think it is mostly a single cylinder issue but it will occasionally get much rougher indicating a multiple cylinder problem. I will check with my cousin to see if I can use his old Sun machine to look at the secondary scope patterns. Thanks for the reply.
I Would current ramp the injectors. If you need help with that go to YouTube on south main auto channel he has videos on older corvette injector issues. Those new injectors could be cheap made in China junk or sat on the shelf for years.
Thanks for the reply. I ordered the set of matched injectors from Fuel Injector Connection. The new injectors gave the car a massive increase in performance but didn't have any effect on the idle misfires. I have recently run a can of castle valve cleaner through the intake vacuum connection to the brake booster. The misfire is definitely happening less but can be felt occasionally. I'm wondering if only having 50k miles in 30 years it may have had a massive amount of idle time and carbon buildup.
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