jimbo1111

I didn't think of you as a pot stirrer. No worries.
I don't believe people here with issues want to revert back to stock and diagnose a situation where there are enough VR systems out there that someone experiencing the same symptoms and rectified them can help. Returning a product is easy. If tech support isn't available why would anybody purchase to begin with?

bradthebold

I am trying to measure the MAF signal voltage, but I'm not doing something right. DME and MAF are unplugged, car to on position. Measuring pin 7 on the DME, I get 0.318V. When I measure the MAF signal voltage on the MAF connector (the blue wire on my connector), I only get 0.005V. Measuring the red wire at the MAF connector gives me 12V.

When I unplug the piggyback and put in the shorting plug, both connectors showed 0.003V.

No shorting plug, the MAF was 0.001V and the analog #1 in at the PB was 0.001V

Droops83

Hi John,

I have been around this forum for a long time, and I know that you have a great reputation, and I see that you are active in supporting your customers. My earlier post was not meant to disparage your product in any way, but was more to point out a general issue with MAF systems on high-boost engines (which of course you could educate me and many others here about till the cows come home).

I hear you about offering remote support, which is why I don't diagnose my customers' issues over the phone! I try to help out fellow 951 owners on this forum because I am enthusiastic about these cars and know that the current owners of them tend to be regular working-class folks such as myself (though some of the prices that pristine low-mileage 951s have fetched of late have given me pause!).

The problem is compounded by the fact that for practical reasons systems such as a Vitesse MAF system or VEMS engine management system tend to get installed during the course of numerous other jobs such as the installation of a larger turbo, injectors, intake reseal, or even a full engine rebuild, which leaves a lot of scope for vacuum leaks or other unintended issues outside the system. Even if this is not the case, typical 30+ year old 951 issues can rear their head and be blamed on the new hardware or software. Therefore, I do not envy this particular role of John@ Vitesse or Peep who makes the VEMS Porsche kits!

After seeing the latest posts of the logs and John's and Tom's additional insight, I would have to agree that a vacuum leak at idle is the most likely cause. Brad, I know you have done a number of high-pressure boost leak tests, which are great for sniffing out boost leaks at full boost pressure, but a vacuum leak affects the seal in question in a different manner, and they also tend to be worse when the engine is cold. This is where a smoke machine comes in handy----they usually only operate at a few PSI if that. The old fashioned brake/carb cleaner test while running also works, but is somewhat crude. If the issue bothers you enough, it might be worth paying a shop a bit of labor time to smoke test the system with everything intact.

Tom M'Guinn

Your piggyback will show the exact voltage from the MAF, and the exact voltage sent to pin 7 after any modifications by the piggyback. Why not just look there? If you are not using the piggyback and need to check with a multimeter, you need to keep the MAF and DME connected and tap into pin 7 of the DME with the car running at idle. Unplugging the DME and MAF and checking with the engine off won't give you any useful information. The MAF produces a voltage proportional to the amount of air consumed when the motor is running and it's that voltage you want to check. So turning off the motor and disconnecting the MAF and DME will prevent your car from producing the very signal you are looking to check. Make sense?

bradthebold

Ok, if I can check with the PB, that is easier. Granted, I still need to check on the MAF side. Yes, it makes a lot more sense to measure while it's connected and running, but how to I measure it when it's connected?

And I will probably get a cheap smoke machine and test through the intake first and least and see if I can find some leak somewhere.

Tom M'Guinn

Great post by a great, long-time rennlister. I guess we all have our reasons for wanting to help. When I was in high school, there was no internet. If I wanted my very beat-up Bondo-mobile to get me to school the next day, all I had was an old Chilton's manual standing between me and a 4 mile walk. And the Chilton's manual covered every car built over like a decade, so my car was lucky to get 10-15 pages of actual diagnostic info. I walked a lot. I would have given anything to have a forum like this to ask questions and get such thoughtful/informed answers like this. I think more than anything that's why I always try to help if I can.

Tom M'Guinn

The piggyback shows you the voltage IN (straight off the MAF) and the voltage OUT (after any changes made by the piggyback). If you have it up and running, there's really nothing else needed to check the MAF and pin 7 voltages. I picked up this smoke tester when working on an old Mercedes and can say it works very well. Smoking mineral spirits isn't the nicest smell, but it works well. Everything on the old Mercedes is vacuum actuated, so count yourself lucky that at least you aren't looking for vacuum leaks in the door panels and trunk!

gruhsy

Speaking about Vacuum leaks has anyone ever used one of these product? I'm thinking about buying one as this seems to be a good idea concerning diagnosing issues such as the idle....my car was doing something similar prior to pulling the engine.

https://redlinedetection.com/

Droops83

Those appear to be professional-grade smoke detectors, which are great but expensive. We have a couple at my shop (not that particular brand), one is 1-3 PSI for evaporative emission system leak detection, and one is ~5 PSI and can find some boost leaks as well as vacuum leaks. They come in really handy and we use them a lot, but they are probably outside of the reach of the budget DIY enthusiast.

I have seen some interesting posts in the past about DIYers using rental party smoke machines and even cigarette smoke, but I don't think I would condone either of these, just throwing it out there.

jimbo1111

My experience is that vacuum leaks raise an idle and not lower it on a 951 in most cases. So we may or may not be searching for the same thing.

Tom M'Guinn

Hmmm... fair point. There may be multiple things going on, and the recently changed transistors on the DME introduce their own question mark. I'd really want to know what the voltage is on pin 7, and see what happens when you increase and decrease it via the piggyback.

jimbo1111

The cut transistor is not an issue. I recently had a good long discussion with Joe from ftech9 after purchasing his dme. Its Irrelevant.

Tom M'Guinn

No, I meant the OP's attempt to change the ISV driver circuit components. Hard to know if that helped or hurt... I think you are referring to the resistor that gets cut to hook up the MAF?

jimbo1111

Ah, yes. Communication error on my part.

jimbo1111

Well I was thinking maybe a higher voltage coil may help . So i ordered a 6 ohm 45k coil . May as well throw some money at the problem. if the fuel isn't atomizing properly this may help. If not than the maf is next in line.