racerog1

I have a 94 LT1 with 70k - car runs great when cool <200 degrees - no issues - but when it warms up the motor starts 'stumbling' and idle speed fluctuates erratically. The coolant temp goes to 230 - which seems pretty normal from what I am hearing for C4s I have not changed coolant in a few years - no coolant leaks or loss at all. I have cleaned the throttle body thoroughly and replaced TPS switch and fuel filter.

can anybody help???

DanielRicany

Are you losing spark when this happens? Or does the spark go from a white or blue color to an orange color?

Is the primary winding circuit of the coil breaking consistently?

racerog1

Im not sure how to check the color of the spark or the coil condition (?)

it feels more like fuel delivery issue to me - I am thinking maybe EGR valve is bad ...?
thx for the reply

Stephenms

I'm having the same issues in my 87. I have been trying to do research but keep coming up empty. New TPS, new idle air with correct idle and timing. Knock sensor perhaps? I do agree with the EGR valve, it takes more vacuum once it opens at running temp. Hoping to find a fix!

racerog1

I took the car in to a local shop and was told the car needed new Opti-Spark and New Fuel injectors estimate $5200 to repair....arrrgh!!! So instead I brought it back home and replaced the spark plugs and wires and guess what??? The car now runs GREAT!!! The total cost for parts was about $100 - whew!!!

Stephenms

Well that's good news! I put new wires about three months ago plugs even longer ago lol and recently did an upper rebuild myself. Still having rough idle issues slightly before 190 but much worse at 200+ I did replace those plugs while I had that bad intake gasket and vacuum leak. Fuel and oil were getting into my coolant. Maybe the plugs could be fouled already? Or not gapped properly? Would incorrect gap make that much of a difference?

Glad to hear that 5200 price tag was just the mechanics shop trying to steal from ya haha

racerog1

It could be the plug gaps - I would check that and maybe just replace the plugs since they are not the easiest things to change - and BTW I was advised by everyone to use ACDelco parts - plugs etc - - so thats what I used.
Just my opinion but yours sounds like injector problem to me (?) How are yours? I have read that it is best to have the original GM injectors 'reconditioned/cleaned' vs buying aftermarket ones - like Bosch or Accel - because the GMs are better made and can be rebuilt. if you look at them it makes sense.

Stephenms

I have AC Delco plugs and wires (7mm). My injectors were inspected and checked out okay. New o-rings upper and lower. When I accelerate I don't have any sign of misfire or unsteadiness until I'm at a complete stop. Even if I'm moving at a roll there's no rough idle. It's very odd. These older Chevys don't seem to like anything but Delco/Delphi. I learned that with my 91' wanna be corvette, the camaro. Haha

racerog1

yes that sounds like what my car was doing too - one thing i never tried was taking it to a chevy dealer for diagnostics - I wonder if they would try and soak me for $5k too??
Very strange beasts these C4s - and those wanna be's too! good luck and thanks for the feedback

Geardo

I never use GM Delco crap in my cars, but something quality like NGK

Stephenms

It's really just preference for parts. I find it easier to put factory spec items back in.

I've been driving around town and noticing a drop in power and idle when I'm sitting at 210 degrees. In Florida I reach that pretty quickly lol. From start to 185 it runs like a dream. Steady idle, good acceleration, tires letting loose from 15 till 45 MPH. But once 190 hits It starts to drop. Idle gets rougher and hesitates. Once I'm at 210 it's like I'm sitting in a sporadic vibrating chair. Not sure if this is spark plugs but what the hell, they're cheap. I did remove three of them today just to look, 56 gap, 32 gap and 25 gap. One prong was bent to the left as well.

Why it would only get worse at 200 degrees confuses me. Maybe the temperature an the way the plugs are so bad has something to do with it. Any ideas?