Rough running 88 Vette

[Ric]

Situation: 1988 Vette, 61,000 total miles since new. Injectors just replaced because of leakage causing starting problems after driving until fuel fully evaporates. Injectors fixed problem. However, car now runs noticeably rough! At idle and at speed, say up to 2500 rpm or so. At high rpms, seems to run fine with lots of power. Fuel pressure test shows all injectors almost exactly same. Monitoring vacuum shows dip in waveform of pressure on cyliner 4, as if a vacuum leak of some kind. However, presurizing cylinder with air and listening through tailpipe indicates no leakage. Killing cylinders individually, one at a time, shows noticeable drop in engine rpm for each cylinder except number 4 which shows very little reduction in rpms. Leak down test does not indicate obvious exhaust valve leak on number 4.

What is probability that valve lifter is bouncing? Would bounce cause this problem?

Does anyone have any ideas on what may be the problem? Problem only popped up right after replacing injectors. Is rough as a corncob. Ideas??

...Ric in Vegas

 

[BigRed]

If you don't have these kinds of tools, try taking an injector off another cylinder and run it on number 4.

Then look at your cap/rotor, number four could be burnt.

If you didn't notice a problem like this when it went into the shop and all of the sudden now you've got a dead cylinder I don't think snooping into the valvetrain is going to help you.

Check the injector, then ignition, THEN run a scan off the ECM it may be indicating a code, THEN SNOOP HARD for a vaccum leak

Check the plenum especially this probably came off for the injector trade.

 

[Ric]

Thanks for the comments. However, ignition scope traces show the voltage during the spark to be right on target and timed exactly correctly. Also, swapping plugs and wires on two of the cylinders still show that #4 is at fault. The injector has not been swapped at this point because of the extensive labor involved in getting to it.

...Ric in Vegas

 

[vader86]

have you done a compression test?

 

[offshore]

You said you listened to the exhaust on the leak down how about the intake? what was the % of leak down test? and did you listen in the oil fill hole for a leak into the crankcase?

 

[95ZR1#418]

I would rule out valve problem, highly unlikely, considering. I would triple check for vac leaks , any wiring which may have been unplugged first.

 

[yellow91]

I also have rough idle in my 1991 vette. I'v done compression check (170-180 on all cylinders), all new ignition componants, timing, checked fuel injectors with voltage tester (resistance is 16.3 on all injectors), also my car doesn't eat a drop of oil, and computer read fine, in fact it reads a top running engine.

 

[Ric]

Mine is rough at idle AND it is worse at speed up to say 2500 rpm. Seems to have lots of power though. Remains a mystery but next week I have getting a second opinon from the Chevy dealer, for what this is worth. At this point, about the only thing left to do is, apparently, to pull the intake manifold off and look for the possibility of a gasket torn or blown out between cylinders 2 and 4 or a worn cam that simply won't lift the valves high enough. Seems like a very small likelihood on the cam but I don't know what else could be happening.