So i'm looking to buy this car locally at the beginning of the week if I can get some solid answers on this issue. It is a 96 z28 with hotcam kit, longtubes, th400 swap (didn't do the speed sensor thing yet) and a few other mods. He said it has not been tuned yet.

Anyway, the issue is it seems the car likes to stall when sitting idle. He said it only started doing that after the th400 swap and has something to do with the vss or searching for idle or something along those lines? (I could be misquoting him slightly) The other main issue I noticed is he has a wideband guage installed and it seems to go from the teens (14-15-16) and spikes up to 25-30) - could this be from not having a tune?

Well the price hes asking for the mods he has isn't too bad, I just don't want to buy a lemon. I'm not trying to get something perfect as I am buying it as a project and something to toy with. Does anyone have any comments on these issues and if it makes sense? I think im more worried about the car stalling out than anything else.

 

The VSS signal is only generated when the car moves. It is used by the PCM for the cruise control and the instrument cluster. It is not used for controlling the engine at idle. It would seem that perhaps the PCM is thinking the engine is running lean and is richening up the mixture at idle to the point it kills the engine. That would explain the rich numbers you were seeing and stalling.

 

But the PCM is looking for the VSS signal to verify that the vehicle is not moving, to allow the engine to return to true idle, and not the RPM dictated by the coast down follower. Whether that accounts for the near stall at idle is unclear, since I don't know what the PCM defaults to when there is a VSS code. I have TH400 and have run with no VSS, and never had erratic idle, but I have a hybrid engine management system that uses the stock PCM for VSS and IAC, and a MoTeC ECU for evertything else.

His wide-band is showing lean A/F ratios, particularly the spikes. That may indicate major misfires, with large quantities of air from the misfiring cylinders passing into the exhuast.

My guess would be that the VSS has nothing to do with the erratic idle, and that its misfiring. The LT4 HOT cam requires a tune, but it should idle OK on a stock tune,

My question would be why does an engine running an LT4 HOT cam need a TH400? Abd, are you prepared to live with no O/D... becaue I can tell you its no fun cruising at 70MPH with the engine spinning at close to 4,000RPM.

 

Thanks for the replies. Well I don't mind the no O/D because this will only be a toy and the only time it will cruise at those speeds would be on the highway driving to the track.

As for the stalling, is it most likely something simple causing it or could it be something major. I don't mind getting my hands dirty because I am buying a "project" but I don't want to go crazy trying to figure out what could be causing a missfire (assuming that is the problem.)

About why he needs a th400 on a hotcam? Not sure, but that is a good question. The car has a few other mods that seem excessive for the build as well such as a holly 58mm TB, 32# injectors(i think), solid link control arms, etc. He says its a stock bottom. Time slips show its a 7.89 1/8mi car.

I know its hard to judge a car without being there to look at it, but if anyone has any advice that would be great.