Blackbird96WS6

Alright guys, looking for some help here. Long story short here is that we have a 94 C4 LT1 that surges at idle once it’s up to regular operating temp (220+ degrees). I’ve been fighting it for over a year now to no avail. Last year during the hot months, I already put new O2 sensors on it, new IAC, new FPR, checked all the vacuum lines to make sure there are no leaks, cleaned the TB and MAF, cleaned the MAP sensor, cut open the air box to let more cold air in, and yet whenever it’s over 60 degrees outside, this thing will continue to surge anywhere from 200-400 RPMs when sitting at idle, either in park or drive. Once winter came, it stopped doing it thanks to the cold ambient air, but alas, that has ended.

Spring is here and warm weather is coming back which means this bitch is back to surging when hot now that there's not constant cold air. I’ve been brainstorming ideas with a buddy and trying to get this thing nailed down. I put a known working MSD blaster coil and ICM on it to see if the old one was the problem and spaced it 1/4" off the head per shbox's instructions. Took it out on a test drive in 80 degree weather, got it all the way up to 233 degrees and it wasn't surging anymore, so I thought "yay, maybe we finally got it." Well, nope... It started surging again just a few days later, not as strong as before with the old coil (it had gotten to the point of having to stand on the brakes at a red light to keep it still and the whole body would jump as it surged). With the new one, it was just doing it about 100 RPMs or so, not much at all.

Well, it ran like that for about a week or so up until this past Wednesday when my wife parked it for the night. Thursday morning she went to leave and it wouldn't fire up - crank, no start. I come home on lunch break and check it out, it cranks over just fine, but wouldn't start at all. I check the voltage readout on the gauges and it says 12.2V. I think hmm, ok, maybe battery's low? Toss the battery charger on it for an hour while I eat lunch, then go back down and test it again. Same thing, still no start, 12.4V on the gauges. Didn't have time to keep jacking with it, so closed it up and went back to work. Couldn't work on it Friday thanks to an emergency fix on the Lumina, our backup/beater car (lower ball joint snapped when my wife tried to go to the grocery store, fixed that after work, fun week hah).

Now we're up to today's events. This morning, I head down to the garage and check fuel pressure – 36 psi when key on and engine not running yet, looks good. Plenty of battery since it turns over just as fast as normal. Except now when trying to turn it over, it'll sputter and make a pretty bad noise, let go of the key to kill it, and then it spits a nice big puff of white smoke back out through the air filter. FML. I go through pulling a plug wire and sticking a lawnmower spark plug in to see if it's even getting spark - didn't see any spark, not sure if it's because that spark plug is just too old or what. I put the wire back on the plug in the block and decide to try swapping the old coil and ICM back onto it and see what happens. Same thing, cranks, sputters, white smoke, ****... At this point, I call a tow truck to come grab it and take it over to my dad's house where all the tools are. Once it finally gets there, drop it off in the driveway, tow truck driver leaves, open the hood and crank it to show my dad what it's doing. And what the f@ck, the crazy bitch started up after a semi-long crank and ran just fine, revving it and idling, no issue. Now I'm pissed and lost for words.

For good measure, I killed it and decided to pull a couple spark plugs just to see if they were showing anything bad going on inside, both plugs 2 and 4 that I pulled looked perfectly fine and were gapped right at .050" like they should be. I put them back in and swap the newer coil and ICM Justin sent me back onto it again. Checked for any codes in the system, no DTCs reported. Pulled the MAF off and sprayed and scrubbed the inside of the TB clean, then sprayed then MAF clean, put it all back together. Try starting it up again to see if it's still going to start up again - boom, back to square one: crank, sputter, nothing, thin wisps of white smoke out the air filter. My dad being used to old school carbed engines suggested pumping the gas pedal while cranking it. Being a fuel-injected engine, I didn't think this was going to change anything, but yet again, this crazy *** car proves me wrong. Pumped the gas and it fired right up nice and strong. Tested this 4 more times in a row pumping the gas while turning it on and it fired up every time, yet if I don't touch the gas pedal, it will just crank and sputter. I call my buddy to pick his brain again and he suggested testing the TPS and seeing if it's getting the right voltages. Looked up how to do that and what the voltage range should be for closed and wide open and it read out what looked to be fine according to some threads on CorvetteForum. 0.585V at closed and 4.0V at wide open.

At this point, I clearly don't trust this thing enough now to try bringing it back home yet because who knows if it'll die out on me en route. I'm out of ideas on what to check, as well. Before we figured out we could get it running, my dad's idea was that maybe it jumped timing by a tooth or something, but since we got it to run by pumping the gas and it sounded just fine and didn't make any angry noises to suggest timing was off, I'm thinking that's off the table. Does anyone else have any insight to this issue now and can point me in the right direction please?

pcolt94

36 psi for fuel pressure is low. Should be about 41 when engine is not running. Should be about 36-38 when idling and when it revs up, should go over 40 for a second.

Try taking a drive with the fuel pressure gauge on the windshield and see what it is while driving. If you go below 35, no good. If it fluctuates a lot with steady throttle, no good.

If there is a fuel flow problem, it will affect the pressure during driving and higher fuel demand compared to idling. (Pump, sock or filter).

You might have more than one problem so I would keep an open mind. The cap on the opti might have carbon tracks causing running or miss-fire problems.

Insufficient spark could be a factor. That could be from any number of factors like ICM, coil or cap. What kind of shape are the wires in. They might have high resistance. There have lots of problems with coils that are not OEM or an equivalent standard coil.

Blackbird96WS6

After messing with it more today, it's looking like the fuel pump and/or fuel filter might be the issue. Tapped on the IAC with a screwdriver a few times, and it started right up. Fuel pressure sits at 30 psi at idle, drops below 30 when revved. Got up to 35 psi under load at 3k RPMs when driving it around the block. Seems to be a fuel delivery issue.

AZBlackC4

I agree. I own a 94 convertible LT1 also, which had a surging at idle problem. Without getting into a lot of detail, I ended up replacing everything you did as well as, the water temperature sensor just below the water pump. The car seemed to stop surging for the most part once it got hot, but once in a while it would surge a little. A few months ago I figured out that my battery was starting to go bad and replaced it. Soon after I replaced the battery, the voltage started to go back down again and dropped below 12 occasionally and the dash lights would flicker. I then, replaced the alternator, which seemed to help immensely. The voltage went up to 14.6 or so, and generally runs about 14.1 to 13.9 most of the time now. The car also runs much stronger since replacing the alternator. Thats not the end of it though. After awhile, It started to turn over more at start up occasionally and eventually did not start immediately after turning over for some time. I waited a few minutes and then it started up. I realized that this was a symptom of a bad fuel pump, since I had experience with one going out a few years ago in another vehicle. I replaced the fuel pump, filter sock, and gas tank seal. Immediately there was a tremendous difference in power and driveability! I think the fuel pump had been very weak for quite some time, contributing the the driveability issues and surging.
I don't think that any of the bad parts had anything to do with each others failure, just that it's an old car and all of the parts you mentioned as well as my battery, alternator and fuel pump needed to be replaced for the car to run well. My car now idles and runs very smooth, and with a lot of power most of the time. I'm going to replace the fuel injectors next because they're the originals. The car runs just a bit weaker and not quite as smooth when warm, then when first started. I know that old injectors can shut down or misfire when hot causing loss of power and some driveability issues. Hopefully it should run close to new after they're replaced.

Hopefully this helps clear up your problems!

PS; I also replaced the fuel pressure regulator and the EGR valve.

Blackbird96WS6

Glad to hear it looks like I'm pointed in the right direction with this. The alternator is still running strong, as it sits right at 14.4V when the engine is running and I haven't seen it fluctuate at all. Going to look at pricing between an AC Delco pump and a Walbro 255 pump and toss in a new fuel filter with whichever pump I go with. I'll report back once installed and let you know how it's acting.

AZBlackC4

I purchased the Walbro 180LPH fuel pump kit from Racetronics and installed it. It comes with the gas tank gasket, filter sock, new wiring and stainless steel bolts. I got the 180LPH because it is rated as a OEM upgrade in pressure vs the 190 or 255, which are both high performance rated for engines cranking 450 plus HP. It doesn't hurt to install a higher pressure fuel pump, it's just more expensive and you don't really need it unless you have a higher HP engine.
I also installed a new IAT. (Intake air temperature sensor). My goal was to identify everything that was temperature sensitive that affected the computer. After installing the IAT, I noticed a big difference in how the engine ran. My total parts list is; IAT, MAP sensor, ICM, Coil, EGR valve, both O2 sensors, water temp sensor, fuel pump and filter, and alternator. I have to note that one of my spark plugs was cracked, which caused a bad surge when all of this started. After replacing that, it still surged but not as bad.
The car hasn't surged in quite a while now, but I haven't gotten the engine temperature up over 215 since replacing the fuel pump and alternator. I'm curious to see if anything happens now, when the engine temp gets over 225. I live in Arizona and the outside temp has been over 100 lately and at least in the 90's for some time, and no surge.

Good luck. I'm interested in hearing the results.

Blackbird96WS6

Well after waiting a MONTH due to the Walbro pumps being on backorder, I finally got it in and just installed the Walbro 255lph pump and a new fuel filter. Tried to start it up and it's got the same issues, crank and no fire, and when you let go of the key it stutters and tries to blow back through the throttle body and intermittently spits up white smoke through the air box filter. Any ideas now, guys?

Blackbird96WS6

https://www.youtube.com/watch?v=oXB4...ature=youtu.be

active1951

Did you run a test on the water temp sensor? I think the other member above replaced his. I don't have manual here now , but normally there is a way to check volts or ohms when car is hot and or cold. I usually end up probing wires to test. Another member might comment about this. If not, I can look it up later.

You might tell your dad this story: My brother had a car back in the carb days, he had Monkey Wards do a tuneup. After that it would not start if the temp was below 70 degrees. He had it towed a number of times, then just could not use it in cold weather. Monkey Wards replaced everything they did in the tune up, supposedly. I got involved, slowly checked everything out. I tried to test the condenser, it acted weird, so I replaced that cheap *** part. Bingo!!! I made Monkey Wards reimburse my brother for EVERYTHING.
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Blackbird96WS6

The coolant temp sensor was replaced with the water pump last summer. What's causing the AIR pump to kick on twice like that when I let go of the key?

active1951

Maybe I'm comming in a lil late on this thread.
Anyway, what does it do when cranking WITHOUT "pumping" the accelerator?
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Blackbird96WS6

In the video there, the first crank was without touching the accelerator. Second crank was pumping it. Was hoping it would spit out the white smoke for me on camera, but no dice. Starting to think it's gonna need a new opti. :/

active1951

opti has entered my mine, but wait.

Without rereading all posts, how old are spark plugs AND plug wires?
Will it start in cool weather, in morning or any other time now?

I would not buy any other parts right now.
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Blackbird96WS6

Plugs and wires were replaced by the previous owner in March 2013. Had the receipts to prove it. Currently won't start at all, but it's summer in Texas. Mid 90s during the day and high 70s / low 80s in the morning.

active1951

This may sound a lil strange, but if you can "cool down" any suspected parts, you can check if it makes a difference.
I have done this years ago, let me think about it.
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active1951

Was it acting up before water pump was replaced?
Why was water pump replaced?
Be sure all wires are plugged onto plugs and all are plugged into opti nice and snug.

Blackbird96WS6

The water pump was replaced because it was leaking from the o-ring in the gear drive at the back. I've checked the plugs being snug, and today we checked for spark (on plug 4 only) and it was good. Didn't check all the others though.

active1951

Was car acting up before water pump started leaking?
Did water get to opti?

I would check spark on all plugs.

cormorant71

i am having a simmilar issue.

when the car warms up it misfires and surges at idle. when it is cold it runs smooth as silk.

the hotter it gets the worse it gets, to the point where when i am up at max operating temp it looses power, stumbles, gets very bad mileage, and stalls.

from what i read, and what i have done, it looks like i need a new fuel pump and fuel injectors

thanks

Blackbird96WS6

Not to my knowledge, ran fine as far as I can remember, but it's my wife's DD and I check the car periodically (how I found the leak). Based on location, I'd be surprised if the opti didn't get at least a little wet, but the car ran for nearly a year after the pump swap. I'll check spark on all the other wires when I can and let you know.