Jeremy Himsel

What I found on mine is when my idle was 14.7 it ran terrible because I had 3 cylinders running rich and 1 cylinder running slightly lean giving me a 14.7 average where the DME was trying to keep it. The swing was +/- .5 though and 14.2 to 14.9. When I richened up the entire bank by adding fuel pressure or SMT-6 adjustments I could get the shaking/stumble to go away but then I had the one previously rich cylinder running okay but the other three were very rich. Individual EGT's would have made diagnosis very easy. Once I found the bad injector my AFR swing @ idle is 14.7 - 14.8.

Fishey

So, you are thinking its injector on time? (What is it set at right now?) Also, what engine management is being used? (Please don't say Electromotive) Also, I don't really care about popular so are the 55lbs siemens injectors saturation or peak/hold?

Jeremy Himsel

He's using the stock Motronic box with a Vitesse chip. When I say "popular" I mean that these injectors are runnning in a couple hundred 944/51 applications and every vendor sells these so it is not a mis match for the system. The 3102's are a P&H low impedence injector.

badass951

Yup. DME w/ Vitesse chip, as I have no reason to go further with upgrades until this issue is resolved. Thanks again Jeremy for the info and sharing your experience.

So what do you guys think? If I can solve this idle issue through the help of Rennlist I will become a paying member again. Seems only fair. I will be trying to find the bad injector and will be replacing it before the Hershey swap meet and if all goes well I should be in Hershey on April 26th. If my issues still remains however I will update this thread and the search for the culprit will continue.

jlturpin

OK, so it runs at 900 rpw with the TPS disconnected. Stumbles all over the place while idiling with it plugged in and idles much lower. I would try a couple of things, if I did not miss them in the previous post.

1. Bypass the Idle control valve, and set idle to spec, around 850 RPM with idle screw.
2. Clean Idle control valve, or did you already replace it?

My car running vitesse is the opposite, it runs super rich at idle. But I am running the 72 or 75# injectors.

I know this is probably to simple to be the cure, but thought I would chime in. 1 last thing, have you actually checked the fuel pressure and flow?

badcoupe

If ypu add propane etc the intake with the car running and it smoothes out then you know that there is a fuel issue with an injector. We commonly do this esp on cmfi systems to check for bad poppets.

badass951

Swapped out fuel injectors and the car does not miss as often, however that seems to be the only change. Still poor vac. numbers and similar air to fuel ratios. The only thing I want to note though is that at the same time I removed the TPS, opened the cover, and cleaned the circuit so when I first started the car after the injector swap the idle was surging up and down until I jumped the plug to disable to ISV. When I first wrote this post I stated I never noticed a change jumping the plug to disable the ISV but for the first time I did. Now when I start the car cold it does the up and down hunting/surging each time until it I disable the plug or it warms up (few throttle blips) and never has before, so I assume it has to do with the TPS adjustment?

Any more advice would be greatly appreciated.

badass951

Also to add, I am going about the proper procedure for setting idle, however I think my adjustment issue is that I messed with the throttle plate stop and can not find the right combination of throttle plate stop and idle screw (TPS adjusted accordingly).

Chris White

I am more of a stand alone person – but here is what I would do. Get an infrared pyrometer (these things are as low as $35 these days) and see what the temp of the exhaust headers is – aim it at the flange since that is not insulated. You can learn a lot checking the 4 cylinders as they heat up. If one is either lagging of way ahead you will have found the issue – could be an injector or the wiring to that injector ( or a slightly bad valve or any number of things). The point it that you can isolate the problem.
I always bring my infrared pyrometer to the track. You can learn a lot when you know the temps – good for brakes, cooling and other issues.
Idle vacuum should be a little higher, sounds like the idle air valve is open to compensate for other issues.

badass951

What kind of issues would cause the ISV to stay open? Also, when you bridge B and C at the test port and disable it, does it just stop where it is or does it close?

badass951

Oh and I will try to pick up a pyrometer I could always use one for work anyway.

badass951

Okay, every time I start it up cold it is surging from 14 in hg to 5 in hg as though the ISV is revving the engine, but it is at low rpm. This never occurred before new set of injectors, but i doubt that could even cause this. Must be the tps...can a bad tps cause surging with the isv? If I clamp the ISV line at the intercooler pipe to the throttle body I feel pressure when the surging is occurring. Once it warms up this goes away.

Why is it that I can understand the engine electronics better on a new Audi or VW and this basic engine management is killing me?!?!

Chris White

A common problem with mods to the 951 is that you have to move the engine bay wiring harness around – its old and has seen a lot of heat. The insulation can break creating sorts, opens and weird resistance. Hard to diagnose and find.
My guess it that your DME is seeing some odd signals and it trying to adjust.

badass951

I agree about the DME seeing odd signals, but not with the wiring, with sensors. I am on my second wiring harness and have repaired this second one myself - 0.2 ohms or less across every wire w/ no shorts present and all connectors seem to have proper terminal connections.

Currently I am focusing some attention on the TPS and throttle body, however I can not find any problems here. Is it at all possible that despite having new ISV it could get stuck if it sat installed in the car and did not work correctly? My previous DME had a fried transistor that controlled the ISV circuit. As I stated for the first time I noticed the ISV causing a surge and pressure in the ISV lines since installing the new injectors, however the car has only been run maybe 20 total miles since the rebuilt DME install as well - coincidence or just odd?

Drove the car last night and it tried idling in the 14.8-15.2 range every now and then and when this happened the idle was slightly higher around 1100-1200 rpm (at the same time vac. would move to 15 or 16 in Hg). I have a lightened crank, anyone ever have idle issues with a lightened crank and flywheel?

Really losing it. Can anyone explain why the ISV would put pressure at the line that goes to the intercooler pipe?

badass951

I need to try a Motronic monitor, it is just too bad Vitesse no longer has them.