94 Z28 rough idle and missing when warm
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Teching In
Joined: Feb 2013
Posts: 37
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From: Pennsylvania
94 Z28 rough idle and missing when warm Hey guys, A few weeks ago my car started missing and hesitating when warm. It also has a very rough idle once it gets up to operating temp. I've done quite a bit of research about it on here but havent found exactly what im looking for. Ill start the car up and its fine until it gets warm. It starts idling pretty rough. It is an auto (unfortunatly..only has 40k original miles on it so couldnt pass it up for the price, possibly a swap somewhere down the line) and when it's in park or neutral its not nearly as bad as when i have it in gear. It starts surging like it wants to take off. And whenever it surges the lights also dim. The part I'm confused about why only when warm? makes me think it's a sensor issue. Been leaning more towards the O2 sensors. Is it possible for them to die that quickly? These problems all started happening at once. Now for a little more background info. Ok, dont harass me for this but i added some octane booster to it just to see if i noticed any difference or not. The problems started shortly after that. Could Octane booster make the O2's go bad? Fuel pressure is fine, Opti is still good, plug wires are good not melted or arching anywhere, havent pulled the plugs out yet but they really dont have that many miles on them. plan to check them soon. Also installed a borla cat back a few months ago. other than that the car is stock. Any help would be much appriciated. Thanks! And i know its long..sorry
If your water pump leaked on the opti just about bet there is your problem.water will kill the opti real quick.Do youself a favor and get a O.E. replacement summit has them i think for 199.The aftermarket ones are junk.
Thread Starter
Teching In
Joined: Feb 2013
Posts: 37
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From: Pennsylvania
The pump is leaking but not that much just yet. I am gonna replace the pump, opti, seals ect. but still trying to find the problem. gonna start with the o2's first. easy and not too expensive. they are almost 20 years old. it only happpens when warm so it makes me think its a sensor issue. but probably wrong.
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If you buy new O2 sensors stay away from bosch they dont work well with LT1s get you some ac delco,denso ,or dephi brand they will give you the best overall performance.Remember if there are any exhaust leaks within a couple feet before or after your O2s your pcm will be fooled into thinking it needs to compensate for a lean reading causing bad drivability and overall performance.Good luck
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Thread Starter
Teching In
Joined: Feb 2013
Posts: 37
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From: Pennsylvania
Yea I've heard the Bosch's are crap. I do have a small exhaust leak in the intermediate pipe. It's two pipes clamped together. Put a Borla cat-back on a few months ago and the clamp is just temporary until i weld it. Nothing serious.
Thread Starter
Teching In
Joined: Feb 2013
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From: Pennsylvania
yea thats why i was thinking that. made sense at the time. after looking into it more it made less and less sense. replaced the fuel filter and ran some seafoam through it. not getting the major problems anymore. actually driveable now. But of course still have problems. still has a rough idle. and almost positive my cat is shot. Thinking about getting a set of long tubes and replacing the plugs and wires while im at it. See if that does anything. last resort is the fuel pump.
check fuel pressure before you do fuel pump. a missing hesitating when warm, id take the icm off and take it to autozone have them test that a few times they've been known to act funny when getting warm. Also don't rule out the opti, my buddy had a 94 that would act up after it warmed up that turned out to be the opti. It was auto so it would rev real high always wanted to just go, and miss like crazy.
id grab the opti and do that first, especially while there onsale on summit, pretty sure thats the cause symptoms sound the exact same, also coolant leaking all over it is no buenos either, i just changed mine this weekend for having a miss.
My dad's 94 Vette did almost exactly what you are describing. It was the opti. His car has slightly over 17K miles. He had the cooling system flushed out in it and they left a hose clamp loose somewhere and he said it steamed the opti. It didn't show that it had lost any coolant, but a light steam is even enough to mess with the opti in these cars...
On The Tree
Joined: Sep 2010
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From: Dunedin, Florida
Everyone is so quick to jump on the opti.... I had a hose that sprung a small leak and leaked on the opti for several trips before I figured it out and after it dried out each time it was fine. I used that opti for another 15k miles before I changed it. I only changed it because a buddy of mine had a spare and I was doing the plugs and wires anyways.
well most of us had have similiar issues and that was the culprit, no reason it should be fuel if its only when warm. the sensor in the optical unit can malfunction under heat, but i gurantee if he puts a fuel pressure gauge on his car the fuel pressure will be just fine, therefore icm is known to act funny when warm, take that to autozone test it make sure they test it multiple time to get it nice and hot. test the coil via shoeboxs website but im sure thats fine too, which leaves the optical sensor in the opti probabl malfunctioning, and like numerous people said they had the same issue and that was the problem.