F18Rep

Morning sport fans...I am working on an 88S 951 with the Lindsey 300 hp goodies. It has an annoying stumble right off of idle and just about anytime you punch it up to around 2 k; the stumble is small but irritating. Also some popping in the exhaust on decel. It boosts and pulls great though.

I've done just about everything in the DME/KLR test plan (except the o-scope stuff - my 40 yo heathkit is just too unreliable - that was old guy humor). The only real lead that I have is the resistance test from the KLR through the knock sensor - it is roughly 2 meg-ohms (bad).

I have the intake coming off later in the week but for now looking for corroborating symptoms and so I marked the PS pulley with chalk for TDC and hooked up a timing light. It appears to be idling (850ish) right at TDC. It does advance smoothly and immediately off of idle. There is a stumble right there so its a little hard to say if the stumble occurs in the absence of advance or in spite of it.

It seems to me like the engine should be idling at 5 +-3 dbtdc not zero - but that is the stock spec maybe not the Lindsey spec. I kinda think maybe, the klr is retarding the timing and that is why it is zero - there is no blink code.

In reading though all the words of wit and wisdom, it seems even the masters are slow to chat about exactly when the KLR retards timing. So if anyone has some suggestions or comments - I would like to hear them...Bruce

ps... this thread has some similar discussion

Adonay

I would check TPS and the idle speed regulator before checking the dme or klr

CDH-CTT

+1... Check TPS...
What about you AFM??? Are you still running the stock one??

alxdgr8

Sounds like the typical AFM driveability. But the timing issue is odd.

F18Rep

Sorry I should have been more clear. Yes, its a Maf set up, yes the TPS has been tested (actually by several people, it and a new ICV have also been installed). It starts cold perfectly and the idle settles down fine.
The KLR/DME test plan is a sort of supplement to the WSM and details a whole series of tests, I have done most of them except those that deal with the AFM, exhaust and those that require a o-scope. Its actually a pretty good turbo 101 document. On this particular car, several non-Porsche professionals have struck out. So I get a turn and thought I would go at it with as much organization as possible. I would pass a link to the "plan" but I only have a paper copy.

The Lindsey 300 rwhp set up is a MAF set-up (maybe he calls it 320, I forget). I think its running the maf firmware load they based it on though as well as a DME chip. I haven't gotten into that (yet). The car is pretty heavily modified in my book - 4" exhaust, stock turbo, wide band, LR wastegate, LR fuel lines. It is a fast car, no doubt... but has a few little things that keep it from being a real street fighter. In fairness, I'm told the stumbling problem was present before the LR stuff was installed.

So you can sort of see that the owner and previous mechanics were thinking along the same lines as has been mentioned (AFM, TPS, ICV). I'm hoping I have the magic fix with a knock sensor and am hoping I'm not chasing another rabbit. And I guess that is why I was hoping someone would have some rememberence of chasing/verifying a timing problem.

Other trivia...My fuel pressure is on the high side (38 and rises to 42 with no vacuum) but again I see a colorful FPR (that is prolly from Lindsey too) and maybe correct for it. The stock injectors read the right resistance, I have the 10 v needed to drive them at each of the plugs and btw the injector wires are still pliable /no broken connectors. Of course I plan to put in the new knock sensor and I still need to check the speed sensor gap (although that seems like a reach). The plug wires also read the right range, the diz cap is fine. Temp2 sensor is correct. I have a spare MAF plus all the original hardware. The wideband seems to move around a lot but maybe I just need some education on interpreting it.

So now, in your opinion/experience is the idle timing that I see consistent with a bad knock sensor?...Bruce

Tom M'Guinn

Since you've run through the regular test, I'll assume all the standard issues check out. I cannot tell you with certainty whether a sensor or harness can fail in a way that causes continual ignition retard, so will just offer a few thoughts. First, it is possible to bypass the KLR and run the car striaght off the DME (I think it's pin 9 and 16, but if you search there have been several discussion). That would tell you with certainty if the KLR is responsible for the timing at idle. Before doing that, however, you might want to just pull the connector off the knock sensor to see if that restores idle timing and, if not, plug the connector into a known-good knock sensor (without installing it to the block for convenience of testing). My only other thought is to confirm that the flywheel is stock and not an aftermarket that may or may not have it's ref sensor trigger positioned properly.

CDH-CTT

When it stumbles what does your wideband say??
Have you check Speed and Ref sensors?? ( pull them out and make sure nothing is stuck to the tips and the wiring to them...)

F18Rep

Thanks for the ideas... An update is coming, just momentarily swamped....B

F18Rep

Its been busy this week so only a small time to look further into this. I haven't really gotten a chance to try any of your suggestions. But I went back and repeated my timing tests to see if maybe I messed something up. Here's a pic of the setup I'm using


So, heres the first clip and shows the engine idling at TDC.

I went back to do it again a few minutes later. Now here is a few minutes later, after a little warm-up time. This seems right, although its a little hard to see, it appears we are maybe 3 d BTDC.

So, I guess, I'm thinking the timing is OK at least as far as I can see in these tests.

Here's a couple of clips that better show the original problem. First, note how far the vacuum falls during the stumble.
And last, just comparison, here is a near-stock engine during the same blip.

One last item... couldn't get the bad knock sensor disconnected with the intake on, so that is off now. The sensor was confirmed as bad. For now, thats were she sits...Bruce