J.C.

Hello together!

I am new to the forum and would really appreciate your help. I am already active in german forums, but as i see there is much more going on here i decided to give it a try
I own a 86' 951 220hp which is bone stock and imported from california (to germany where i live).


I've been trying to diagnose and solve a problem I'm seeing while accelerating with about 50-60% throttle at about 3000rpm just hitting 1.05 to 1.1 bar at the boost gauge. With these conditions, the car seems to studder and hesitate, as if the engine can't decide whether to give it gas or not. I've noticed on my wideband, that the problem occurs just when the engine goes from closed to open loop.

There are following cases:

O2 sensor connected:
-Engine hesitates when accelerating (worst just after cold start)

O2 sensor disconnected:
-Engine accelerates very smoothly

When i started to hunt this problem down, it was much worse. Meanwhile it is barely noticeable but drives me nuts....

Here is what i changed so far:

- FPR, Fuel Dampener
- Fuel Pump, Filter, Lines
- many many vacuum hoses and pipes
- AFM, TPS
- ignition cables, plugs, rotor, cap
- blow off valve
- reworked injectors
-changed dme with a known good one

I already found many vacuum leaks, but i think my AOS is leaking as well. Could that be a possibility?
Also i notice a fluttering noise as the boost is starting to come on....

I'm looking forward to your ideas!

Dwizle

I guess the next thing to change is the O2 sensor or check the wiring from the plug to the DME

928 DesMoines

HY JC. Where in Germany are you ? I'm in Kaiserslautern.

ElRicardo

Yes, you might try changing that O2 sensor, it’s cheap and easy compared to the other things you have changed so far- I didn’t see that you did. My ‘87 turbo has the exact opposite problem- rough until the system heats up enough to go to open loop. Changing the O2 sensor made no difference, but in my case the “cure” is just wait a minute or two before getting on the throttle.

J.C.

Already changed the O2 Sensor, as well as bridged the wiring directly to the Dme.

sorry I forgot to mention...

i have a quite similar symptom as well. When the Car is cold, I mean really cold, at least sitting over night, the throttle response and overall performance of the engine is pretty bad until it gets a little temperature after 1-2 min of running.

There might be a connection between these issues?

I’m from nearby Stuttgart

928 DesMoines

I'm chasing something similar. DME temperature sensor?
Conflicting threads on exact location for a 951. Pelican points to a single sensor next to the throttle body with a single wire connect. However, Rennlist discusses a two prong sensor in a similar area.
Been trying to figure this out lately. The part itself is inexpensive either way; and am thinking I'll do it as a "process of elimination".

Anyway, your latest post is exactly what I am trying to fix on my car as well.

ElRicardo

I had changed the DME temp sensor as well- it is cheap, like $20, and easy to get to. No difference. So that wasn’t it. I swapped in a known good AFM. No difference. That is about as far as I got. So if you do make a breakthrough, keep me posted!

J.C.

Forgot to mention I changed the Dme temp sensor as well 😄 that was one of the first things I tried.

through the whole process so far the best „gains“ I achieved by eliminating a couple of small vacuum leaks.

i know there is another small one left on top of the AOS, sure it is a good idea to fix that as well...

my next try will be the cycling valve since I got a tip from a friend that these are always leaking a bit.

what are your thoughts on these possible issues? Somone cured similar symptoms with these parts?

J.C.

what do you mean by conflicting threads?

me no good english 😄

ElRicardo

I think maybe the conflict is that there are 2 temp sensors near each other. The one with a simple two spade connector is the temp sensor that send temp information to the instrument cluster. The other with a clip on top is the DME temp sensor.

J.C.

Yes, there are two sensors. The one with the blue connector is the DME temp sensor (which i changed). The other one just feeds the instrument cluster. The third one with one single wire nearby the throttle body is actually a switch which operates the additional electric waterpump for the turbo.

Any comments to my AOS and Cycling valve ideas?
what's still strange: with a disconnected O2 Sensor the car accelerates smoothly...

Droops83

I would concentrate on this issue first. It is possible that you have a large vacuum leak, which will always be most apparent on a cold engine when an extra-rich air/fuel mixture is needed.

Perhaps you should take the car to a shop with a professional smoke tester? This can be used to check the intake manifold, breather system/AOS (by pumping smoke into the oil filler neck) and boost pipes.

This would also explain why your engine hesitates with the O2 sensor connected as the engine is warming up. The factory oxygen sensor will have almost zero effect until the oxygen sensor is heated up (both by the exhaust gases and the built-in electrical heater), and will not have an effect on the air/fuel ratio for at least a minute or two. But, once the DME does enter "closed-loop" mode and consider the O2 sensor input, it will have an effect on overall running. This old-style, "narrow band" oxygen sensor is only accurate around the stoichiometric air/fuel ratio of 14.7:1 for a gasoline engine, so if the actual AFR is much leaner, the DME will attempt to adjust the AFR by crude means, which will often result in the stumbling that you are experiencing.

black944 turbo

I had a very similar issue for 15 years and tried all of the items you mentioned. I finally figured it out about 4 years ago and it was a damage knock sensor harness. Once that was fixed the car runs unbelievably well. I would suggest checking that as well. The wires can easily get damaged in that area.

BTW, even the best running 951 is not as smooth as a modern car.

J.C.

Thanks for your input guys. At the moment I have a wideband connected, which shows me that in my stumbling conditions while accelerating the Dme tries to switch maps. It looks like the Dme is alternating between closed loop and WOT maps.

J.C.

@black944 turbo that’s very interesting, since I already found a brittle harness at injector No. 1 which I fixed. Chances are good the rest of the under manifold harnesses are pretty much toasted too...

At the moment I have to work on other projects but I’ll keep you updated!