Dale1990

My 90 is having a problem. In the morning the car has a really bad hesitation / stumble. I can barely get the car to launch w/o stalling. When it warms up it is better but it is still a little rough. I have replaced the plugs(Delco RapidFires)/wires/coil/cap/rotor so for with no success.

When I pulled the old plugs out (Bosch Plat) they looked pretty bad for only 10K miles. They were pretty carboned up and had light colored "ash" on the ground strap.

Any ideas?

Thanks
Dale

gpagaduan

I would guess that it's an IAC valve problem but since it goes away after the car is warmed up I don't know. Are you sure you dont get any idle surging or stumbling when your approaching a stop? A no cost fix would be a throttle body cleaning. Good luck.

Dale1990

You might have something there. When hot it will sometimes surge and try to idle at like 300 RPM or so. Hmmm... I forgot about that. I guess my next step is to pull that thing apart tomorrow and give it a good cleaning.

Any other ideas?

I am still a little worried about the condition of my old plugs but I also do not trust these Boschs so it might be inconclusive.

Thanks!

gpagaduan

For $20 or 30 bucks at autozone you can replace the IAC valve. If you dont mind possibly taking the TB off twice, you can clean the TB and see if that solves the problem. Take off the IAC valve and clean the pintle too. If not, replace the IAC valve.

Dale1990

I'll try cleaning the IAC in the morning (should be done anyway) but I don't think it is the problem. When I took the car out this afternoon, it is real sluggish and seems like it is bogging when driving under load. I am pretty sure the IAC has little affect when under load.

Thanks
Dale

RED-85-Z51

had similar problem just recently. New plugs, Wires, set TPS to 0.54 or 0.65. 0.65 is the performance adjustment.

Im not sure if you have an adjustable TPS though, so Id see if it wasnt a faulty TPS.

My TPS was off, and it screwed my plugs up so bad that carbon had bridged 1 plug, and the others were nearly unrecognizable as sparkplugs. I had 4.99ea. Rapidfires, and they did okay, but only lasted me 10K miles or so. So I put in the cheap Delco stock plugs, and cut back the ground electrodes, runs a whole lot better!

gpagaduan

That's probably the only sensor on my 90 that hasnt been replaced. It's not adjustable...how do you test it?

JOliver

I went through this a month ago with my 90 Coupe(110K) I replaced every known and unknown problem with it and it ended up being my computer...I went to advance and got one popped it in a ended months of frustration.Just an idea the computer was under a hundred bucks You might wanna keep it in mind.


Jason

Dale1990

Well, I checked the TPS and it is at .63 and seems pretty smooth.

I really doubt it is the IAC because it happens under load and the idle is normally fine.

It could be the ECM but I think I would like to look elsewhere first. I checked the car w/ DataMaster and all of the sensors seem to be doing OK.

This problem feels like a bog or something. Since I have replaced most of the ignition system, I am leaning towards a fuel problem. The fuel filter is new.

The injectors have 130K+ miles and to the best of my knowledge they have never been cleaned. Does it seem reasonable that they could be the problem?

Any ideas?

Dale

Steel Blue 91

Arrest Me Red

A few thoughts:
You might check your fuel pressure. Is it in the low to mid forties without the vacuum connected to the regulator? Do a leak down test to see if pressure holds for sometime after shutting off the pump? If it doesn't, you may have leaking injector(s). A bad regulator or fuel pump may result in your symptoms. Checked for vacuum leaks? Are all your injectors firing? Check resistance across each injector's contacts. It should be in the range of 15 to 16 ohms. Unplug each one and check resistance on the injectors. You have an injector that is trash if you have no resistance. Are all your plug wires providing fire? How's your distributor look inside? Is everything together like it should be? No loose rotor screws?

That's all for now. Good luck.

Dale1990

The cap/rotor/coil/plugs/wires are all new. The old ones looked pretty bad - a lot of "stuff" on the points under the cap and the coil had a funny burned electic smell.

The fuel pressure is in the low-mid 30s w/ the vac line connected so it should be in the 40s w/o the vac line. I did not check to see if the rail held pressure (why, I cannot imagine - I know better).

I need to head to AutoZone and pick up a gauge but I am stuck on third shift until Mon. so it might take awhile.

The thing that confuses me is that the problem almost goes away when warm. I runs rough-ish but not anything like when cold.

Dale

kyle87vette

i have had a similar problem and with bosch plugs incidentally but they were not the problem. the obvious things to check are fuel pressure(if runs better with vacuum off fuel pressure regulator you are running lean) if possible hook fuel pressure guage up so you can see it while driving. if it drops at wide open throttle the fuel pump is problem. i changed all distributor parts (plugs,wires,cap,rotor button and coil and none fixed problem. I ended up changing distributor itself and it fixed the problem.

Sam Lam

I would suspect injector problems. Check the resistance of each injector and this should be in the 16 to 17 ohm range. Check the resistors while the injectors are cold and when they are warmed up. If your injectors are a medium to dark gray, you have GM Multechs which have a reliability problem. Readings below the above range indicate the internal coils are shorting out. I had three shorted injectors in my 89 with 40,000 miles.

Dale1990

I'll try to check the resistance of the injectors when I get off work in a few hours when the car is still warm and again before I leave for work today (cold). That should narrow things a bit. They are the gray-ish ones so that might explain a lot. Some of them were caught up in my engine fire a few years ago possibly speeding their demise.


Thanks!

Dale

Dale1990

Ok, I'm back. When warm the injectors measure 17-18 ohm and when cold they measure 16.5-17.5. Does this mean anything or is it inconclusive? Could it be they are just dirty or should I be looking elsewhere?

Any ideas?

Dale

Arrest Me Red

The resistance tells you your injectors are good electrically.
Check for vacuum leaks and fuel pressure with the vacuum line off.
Fuel, air, spark.

Dale1990

OK, I'm back again. Working 3rd shift really screws w/ my hobbies

Anyway, here are the results.

Fuel Pressure w/ Vac: 35
Fuel Pressure w/o Vac: 46
Engine Vacuum: 17-18inHg

Fuel pressure leaked from 36 to 20 in ~1hr. I know there will be some leakage but is this excessive? I am leaning towards dirty injectors as the current problem. They have 130K+ on them and have never been cleaned.

On a positive note, I cleaned the TB paying particular attention to the IAC and now the car's idle is much more stable but still has the "bog" (not as severe now that the IAC is working) while cold and a little bit of irregular missing at idle.

Thanks
Dale

splehigh

I know this is an old thread, but did you ever figure out what was causing the bog?

I'm having the same problem with my 90.

Thanks,
Steve

Dale1990

To be honest, I don't remember and I'm surprised I didn't come back to this thread to update it. Sorry about that. Wow... this was 50k+ miles ago.

I did eventually replace the injectors with LS1 take-offs from SLP (very cheap at the time) but I am not sure if that was the fix. There have been a lot of other repairs since then.