m50b25 revving between shifts

[camshaftgsxr]

so when driving my e30 m50b25nv swap stock everything with a shaved m20 flywheel and basic cone air filter, im getting some weird drivability issues that I cant find any information.

When going through the gears and loading the engine the car drives fine until I shift, when I shift, lets say from 2 to 3 the engine revs slightly and holds rpm for a good 2-3 seconds and then falls back down very slowly, if I don't put the car back into a gear it will fall back to idle and dip just slightly but not stumble and go back to an idle, which I feel is rough compared to my 272/272 cammed m20.

any suggestions on where to start?

[e30austin]

Vacuum leak.

[camshaftgsxr]

Don't see how, every intake gasket and boot/hose is new, I'll smoke test it and see it I find anything.

Thanks.

Edit: I do not have a one way valve on the brake booster, could that be the cause? I don't have any idle fluctuations when I depress the brake pedal.

[Wh33lhop]

That could contribute to it a bit. Have you adjusted your throttle cable correctly so the throttle body is shut against the stop when you're off the gas?

[camshaftgsxr]

yes, I double checked that first along with the cruise control cable and it is snapping shut immediately. also cleaned the iac profusely. it only appears to be happening under loads, like 50% throttle accelerating, not while cruising down the interstate. still have not smoke tested it yet.

[camshaftgsxr]

ok, vacuum tested it last night and didn't find anything.

[meanEG]

Do you have a speed signal going to the DME? Cable too tight possibly and engine mount flex is pulling on the cable.

[Roysneon]

Try running a spring with hooks on the end (like rear brake hardware style) on the tb plate to force it shut. Mine looked like it was completely closed but was doing the same thing, but only occasionally.

[camshaftgsxr]

Do you have a speed signal going to the DME? Cable too tight possibly and engine mount flex is pulling on the cable.

couldn't be engine mounts they are condor hard poly mounts. can you explain what you mean by the speed signal? im sure I know what it is but im not understanding what you mean and drawing a blank.

Try running a spring with hooks on the end (like rear brake hardware style) on the tb plate to force it shut. Mine looked like it was completely closed but was doing the same thing, but only occasionally.

I could throw one on there and see, I think ive got one at home.

[tonerrrr]

My buddy's 350z did that and it made the car really annoying to drive. It did it even if you blipped the throttle at a stand still. I just thought that was how it was... Have you done a stomp test or can you do them with the swap? Don't rule out the MAF either, they do weird things when they're going.

[justin_l_]

Speed signal= Vehicle speed signal (VSS) search it, there is a bunch of info on it. It should be wired through the c101

[camshaftgsxr]

for anyone searching, I replaced the tps which smoothed out the idle a lot and put a one way valve on the booster, those two items solved the problems that I was having.

with some brainstorming we feel the issue was caused by the booster essentially becoming an unmetered air reservoir and holding air taken in through the throttlebody without letting it get into the engine, this was possible because of the manifold pressure drop during accel, and caused the engine to run slightly rich when the throttle was snapped shut, then the air would rush into rich cylinders causing a slight rev.

that's what we came up with at least. moral is run a check valve they are cheap.