1990 Olds 88 shudders at speed

[Moodie]

I just bought a 1990 Olds 88 Royale (3800cc V-6) on the cheap. The car runs well considering its age except for one thing. When driving on a high-speed road (parkway, etc.) for 15 minutes or so (that is, once the engine warms up) it will start to vibrate (shudder) when accelerating. It only does this when I lightly press the gas pedal to adjust my speed. It doesn't happen during vigorous acceleration or while coasting. After it starts doing this the vibration happens at slower and slower speeds, going all the way down to about 40MPH. It's as if the problem is caused by heat causing something near the engine block to malfunction. The seller told me he replaced the fuel pump. AFAIK it's had no other recent repairs. Whatever it is it's not lighting the "check engine" light. Anyone have any ideas what it could be? I was thinking that it might be bad spark plug wires. Could it be connected with the cruise control (which I never use)?

[maxwedge]

This " shudder" can be an engine misfire when loaded such as in o/d and lock up, when accelerating the trans downshifts and eliminates the load.

[Moodie]

Originally Posted by maxwedge

This " shudder" can be an engine misfire when loaded such as in o/d and lock up, when accelerating the trans downshifts and eliminates the load.

Wouldn't a misfire happen even when the engine was cold? It only shudders when warm and it does it even when the gas pedal is pushed very slightly. Could spark plugs be the cause or do you think it might be something more serious? Also, it's not idling as smooth as it should and pickup from a standing stop seems a bit sluggish.

[niemo810]

I drive a 94 non supercharged 88 LSS. i have the exact same problem.. just resting my foot lightly on the accelerator causes the car to shudder. confuses the hell out of me and all of the other mechanics ive talked to. theyve suggested things like O2 sensors, coil packs, mass air flow sensors, etc. but if any of these things were bad, you'd think it would happen all of the time. but no, its only when the engine is warm.

today i bought some carb/throttle body cleaner and sprayed the crap out of the throttle body (after taking the honeycomb filter off) and a lot of crap ran out. tomorrow i'm going to try some professional injector cleaner (not the stuff that goes into your tank).

i'm thinking that because materials expand when warm, the crap on the injectors is expanding and preventing a steady flow of fuel to the engine.

but if this was the fact, then why wouldnt it do it while idling?

i have no idea what's going on.

if i get to the bottom of this i'll be sure to post as soon as i know.

by the way, assuming you have a temperature gauge, what does you car usualy run at? my car runs between 200 F and 280 F (just under the start of the red line, which i think is very high. my coolant is good and both my fans work.)

any help is appreciated and i'll help as much as i can

[Moodie]

Originally Posted by niemo810

I drive a 94 non supercharged 88 LSS. i have the exact same problem.. just resting my foot lightly on the accelerator causes the car to shudder. confuses the hell out of me and all of the other mechanics ive talked to. theyve suggested things like O2 sensors, coil packs, mass air flow sensors, etc. but if any of these things were bad, you'd think it would happen all of the time. but no, its only when the engine is warm.

today i bought some carb/throttle body cleaner and sprayed the crap out of the throttle body (after taking the honeycomb filter off) and a lot of crap ran out. tomorrow i'm going to try some professional injector cleaner (not the stuff that goes into your tank).

i'm thinking that because materials expand when warm, the crap on the injectors is expanding and preventing a steady flow of fuel to the engine.

but if this was the fact, then why wouldnt it do it while idling?

i have no idea what's going on.

if i get to the bottom of this i'll be sure to post as soon as i know.

by the way, assuming you have a temperature gauge, what does you car usualy run at? my car runs between 200 F and 280 F (just under the start of the red line, which i think is very high. my coolant is good and both my fans work.)

any help is appreciated and i'll help as much as i can

Nope, it's a '90, no temp gauge, no nothin' (except a speedo, a gas gauge and a buncha lights). A real downgrade from my previous car: a 1990 Honda Prelude (oil pressure & temp gauges and a tach) (ICYW, the timing belt broke).

[Moodie]

Update on my 1990 Olds 88 Royale for niemo810 and everyone else:

Took car to repair shop where I bought it. Problem turned out to be a bad ignition control module. It would work okay while cruising but would crap out whenever any kind of load was expected of it. Car would accelerate only when gas pedal was pushed strongly enough to downshift into a lower gear. Module was replaced ($170) at no charge since I had wanted shuddering fixed before I bought car. Car ran fine after that - nice & smooth - but a few days later the AMP light started coming on, the dash lights dimmed almost to nothing and the electric windows became as slow as glaciers (not that they were very quick to begin with - my father says GM cars all have slow window motors). Soon after that it stalled out and wouldn't start. Seems the alternator went (maybe damaged by bad ICM?) and it was running off the battery for a bit. Had a rebuilt one put in about a week ago and have had no problems since. BFN and thanks to all for your help.

[niemo810]

thank you for the reply moodie. I'll try replacing the ICM in the next few days hopefully. by the way, did your gas milage improve any when you replaced the part? thanks again.

-Rob

[osoblanco]

Had the same problem on my 1986 98 Regency, and I just found the cause - broken #1 spark plug! Cost $2.50 to fix, I'll be chortling for weeks. Must have bumped it when I took off the exhaust manifold to change the starter, almost two years ago now. Don't recall that this was a problem until last summer, but it was getting worse and worse recently until I was finally motivated to do something about it. Spark plug boot kept things more or less in contact, so things ran OK except when the engine was asked for a bit more.
Great forum guys.

[niemo810]

Originally Posted by niemo810

thank you for the reply moodie. I'll try replacing the ICM in the next few days hopefully. by the way, did your gas milage improve any when you replaced the part? thanks again.

-Rob

hmm, ok.. well i still haven't replaced the ICM, and i'll tell you why. first, i don't feel like shelling out $170 for an ICM when i don't even know if that's the problem.

second, the problem is intermittent. i was sitting in my driveway tonight, resting my foot on the pedal, and it was chugging and spitting while my foot was stationary. i got out of the car, opened the hood and tried to make the car act up by moving the throttle right on the throttle body. no luck, wouldnt act up. i went back in the car and tried doing the same thing with the pedal that i have been doing. nothing, normal operation.

in 10 seconds, my car went from chugging and spitting to normal, and i didnt change a thing.

keep in mind that the idle is always good, it only chugs and spits when RPM's are between 1200 and 2200. high rpm is perfect and so is idle. also only acts up when warm (but it doesn't always do it when warm)

it sounds like a bad ICM to me, but then again.. why intermittent?

i hope i'm not just repeating what everyone else has already said, but i'm really trying to get things straightened out before i end up spending money i dont need to. i'm completely confused and any help would be greatly appreciated.