My other thread:
http://forums.corvetteforum.com/c4-g...when-cold.html

I replaced the CTS, and it still runs like a POS when its under 180 degrees. Could this be a clogged cat/thermostat stuck open?

Please help its really pissing me off.

Checked the injectors, all checked out @ 16 when hot, 17 when cold.

FP is right there at 39, through the rpm range (when its warm i can get the rpms up, but its at 39 when its cold)

I would look at the open/closed loop issues on something like that. Good Luck

Mike

Sounds like it is too lean. What modifications have you done to the car and what are the operating issues that need to be addressed? Explain what actually happens and when it happens. Does it run OK when driving at a steady speed, when slowing down, when accelerating? These answers will help us trouble shoot the problem.

I live near you, it may be easier to call me and I can try and help you with the problem. 920-627-0156

Brian

the car is all stock besides a k&n filter and a cut lid, and tb bypass. When I start the car, it idles fine but if i touch the gas it backfires through the intake. If i put it in gear when its cold, it will move but touch the gas and it backfires. Once its above 180 degrees it revs good, and on the highway not a problem.

The backfire would make me suspect the airpump or a cold start injector malfunction. Make sure the header checkvalves are intact or just clamp off the hoses to the headers and see if it goes away. Disable the Cold Start and see what happens. Otherwise, scan it to see what the long term fuel trim is.

how do i check header checkvalves? what do you mean hoses to the headers?

I had the same symtoms with my 1990 last Aug. It just started. Went thru all the checks as you did above...nothing....found by accident that the bolt tie down on the distributor was slightly loose and my initial timing had worked down to 0*...reset and all was well again...

Air pump directs air to the headers at start up to light off HC's. Too much unburned fuel, like when you rev it up, and the extra air will ignite the mix and cause a backfire. System is designed to cutoff the flow - either with vacuum or electronically, to keep this from happening. And to keep air from being sucked into the headers - exhaust flow creates negative pressure - each has a checkvalve that only allows the air to be forced through them with positive pressure from the pump. Those valves are screwed into the front of each header and a rubber hose is attached to them. You can clamp off the hoses to see what happens. The checkvalves are tested by unscrewing them and blowing air through them. It should only flow in the direction of the headers. If clamping off the hoses cures the problem and the valves check out, something is up with whatever controls the flow from the pump. Note, when the engine enters closed loop operation, air is redirected to the CAT and your system seems to be ok once that happens so it's more probable that something might be wrong with whatever controls the flow of air. Clamping off the hoses is a simple way to find out.

HC's = hydro carbons?

And by headers to you mean intake manifold on a stock car? I looked at the intake manifolds on my car and i didn't see any little rubber hoses going into them. Could you take a picture of them and send it to me or post it on here so i can see exactly where they are?

Thanks

Interested to hear the answer to this one...

My 89 A4 Coupe just recieved a piggy-back on the PROM to: Disable VATS, turn both fans on at 160 (AZ),
and change the fuel injector modulation to adjust for my 24lb Motorsport injectors. The first two issues are
absolutely beautiful, my car runs at a nice temp, and no more VATS headaches, but now it runs poorly until
closed loop, and then not as nicely as before. It DOES indeed feel lean, and I'm tempted to pop that thing
out.

You want the exhaust headers - there's one on each side of the block, bolted to the heads. The hoses run to the front of each one.

Shifter: Best way to confirm a lean or rich condition is with a Scanner. Long term fuel trim is stored in memory - you want to see what it is if the problem is occurring in open and closed loop. The above problem seems to be an open loop situation only and the location of the backfire points to the airpump. Yours sounds like the incorrect tune. Start with getting rid of the piggyback and using the original tune. It shouldn't have any problem correcting for 24 lb injectors. If your original setup can't run it, you've probably got an air/vacuum leak from the injector install and if a scan confirms it, that's where I'd look first.

I've been having the same problem. small backfires in the intake and runs like crap til about 170 degrees. I 've checked everything except the cold start injector. Isn't it under the rear of the plenem??