Tyler Rose

Hi everyone,

Back in October, I ventured into the 951 world with my first purchase - a 1987 944 Turbo. I bought the car with few service records and from a selling dealer that knew little about the car, however I fell in love with the setup and drive, so decided to move forward at the right price.

After addressing much deferred maintenance, the car was running well, but the mixture always felt to be too rich. The car has a wide band sensor and indicates an A/F ratio of 10-12.5 when idling, and <10 under full load. To my knowledge, the car has a wastegate spring as well as a Lindsey Racing chip. It’s peak boost is 15psi.

A few weeks back, I was driving the car to my shop to have new tires mounted and all was well until a WOT, full boost pull at highway speed. My aftermarket boost indicator displayed 10psi, the car then cut fuel or spark (or both) as if it hit a redline, and coughed out black smoke (unburnt fuel). My initial conclusion was a vacuum/boost leak had sprung, as I had an issue boosting past 10psi previously, and it was simply a loose clamp to the throttle body. I pulled the car over, and on initial observation determined that all clamps appeared seated and tight and could not hear any sounds indicating a vacuum leak. I then proceeded to bring the car to my shop, where the tires were mounted and limped the car home. As I drove more miles, the mixture appeared substantially more rich, to the point where my AFM would not provide a reading, but just “—-“ and would not pass 10.

The following day, I drove the car to my mechanic (Rennwerke in Elmsford, NY,) where they were going to address some issues with work they had done previously. Cheech (if you know, you know) gave me a call after addressing the other issues stating the car was running extremely rich and he’d believed it to be a bad head temp sensor (another part they’d replaced during the major service.) He later notified me that the mixture was still not right, and that the fuel system tested fine, so he believed the problem to be a leaky injector. I then sent the injectors out to Marren Fuel Injection in Connecticut to be rebuilt. Upon installing the injectors, Cheech stated that it had fixed the idle mixture problem and the car was running fine once again. Rennwerke believed this to be related to my overboost protection issue.

Upon picking my car and driving it home yesterday, I determined the car to be running better, but not great. I still do not have a perfect mixture at idle, and there’s slight hesitation when returning to idle in neutral. In addition, I still cannot get the car to pass 10psi of boost without the engine cutting fuel/spark. I do not hear a BOV-type sound when it cuts off and it is precisely at 10psi each time. I have not weighed out a vacuum leak, however I believe it to be the DME/KLR protecting the engine. I have not yet tested the MAF sensor (stock and likely original) or tried bypassing the CV, although Rennwerke weighed out both. I am looking to get the vehicle ready for some DE days this season, so I would like to address this problem soon. Any due-diligence is much appreciated.

Apologies for the long post. I have been a long time reader of Rennlist, but have never posted before, calling for a brief introduction.

Thank you all, and I hope to see/meet some of you at the track this season!

Tyler

RGS

I am just barely getting my feet wet with these cars but have you pulled the computers to verify the correct chipsets are in there? Still running stock AFM? Stock cycling valve? Or manual boost? Can the oxygen sensor richen up the mixture that much? Seems like you may have a couple issues. One being fuel and the overboost or KLR noticing something and shutting down the boost (if you have stock cycling valve)

Breakaway944

You have a car with an a/f gauge and aftermarket boost gauge. Immediately says highly modified to me.
Why did they rule out both? Is it because 951s do not have a MAF, and when modified to run 15psi do not have a cycling valve?
You need to figure out what controls your boost - a black **** or an ebc. Figure out all the mods done to the car.
Before you load up the parts cannon again test, do not listen, for air leaks.

Tyler Rose

My next step is to pull the DME and see exactly what’s going on in there. The car has spent much time in the shop and little time in my possession. The AFM is stock, CV is stock, no MBC. I definitely believe it to be computer related.

Tyler Rose

I have both a MAF and CV. Maybe I should test a bypass to rule them out? I am clear on the mechanical mods to the car, but nothing computer related, so I do plan on pulling the DME this weekend to see if it’s a bad time, etc. These cars pose quite a learning curve, which is twice the stress but also twice the fun. I suggested smoke testing to my shop but they did not see it being necessary.

Porvair

I would suggest checking all of the vacuum lines very carefully. I had a line (connecting the blowoff or bypass valve) pop loose last year and that had a very noticeable impact on boost. 30 years of high underhood temps can take a toll on these lines.

For what its worth, I have heard very good things from friends who use Cheech. I have heard that he is top-notch.

Best of luck!

Tyler Rose

Yeah, I’ve had that hose blow of due to a loose clamp as well. The next step I was going to take was to pressure test/smoke the car, however the symptoms appear to be consistent with a computer related issue. My previous experience came with an atmospheric BOV sound due to the leak. I am not ruling out a vacuum leak completely though.

Dan Martinic

I would imagine the mechanic(s) have checked all the basics. From your posts, it's a little confusing if you have the stock AFM (Air Fuel Meter) or an aftermarket MAF. If you have a MAF, you should imvestigate which setup including chip (Vitesse, Rogue Tuning, Lindsey, etc).

FYI I had a little vac line completely open and my boost wasn't affected at all. The system can overcome leaks in many of the small lines (not the boost-related ones obviously).

Tyler Rose

Sorry for the confusion. The car has a stock MAF sensor with an A/F mixture gauge. That is what I meant by AFM.

Dan Martinic

The stock 951 does not have a Mass Air Flow meter.

Anyway... make a blink code tester, go for a drive, and when it cuts out while boosting, pull over & follow the directions for its use. Example code reader: https://www.lindseyracing.com/LR/Parts/BLINKCODE.html

Instructions for use: page 22 of this very useful factory test guide http://arnnworx.com/download/DME_KLR_Test_Plan.pdf

Tyler Rose

Thanks. I will try to get to it this weekend and report back.

As far as the air flow meter goes, I am speaking on part 951-606-121, which to the best of my knowledge is on every 951 I’ve seen. Please correct me if I’m wrong.

Dan Martinic

The stock unit is an AFM (Air Flow Meter) which gets its airflow signal by means of a moving flap door attached to an arm of sorts moving along a resistor track. First used on '70s Beetles I believe. This is very different from a modern MAF (Mass Airflow meter) that uses a heated wire to relate airflow.

Just a terminology thing... but around here, it helps to use the conventional nomenclature.

I am curious about one thing though: you seem to have stock components and are reading 10psi boost--which is roughly stock. In pre-88 S turbos, this drops to about 7psi at high rpm. What do you mean by the "cutting out" at 10psi?

Tyler Rose

Understood.

The intake side is completely stock, however the car had an external wastegate with a (no longer) atmospheric vent and a modified spring, yielding a peak boost of 15 psi. Currently, when the car reaches ~10psi, it violently sputters, backfires, and does not allow boost to increase.

divil

I don't know if I can shed much light on the problem, but here's some info about the boost/overload protection:

The KLR measures manifold pressure, and makes small, smooth adjustments to boost to try to track the pre-programmed curve. The DME does not measure boost, but it does have an overload protection feature - it has an maximum safe airflow/rpm curve and if the actual airflow exceeds that curve, it cuts fuel suddenly, like the rpm limiter. I don't know if that's what's happening, since with a stock turbo it should not happen unless you have really exceeded the stock boost curve. But, since you are having other issues, I'd recommend testing the AFM output and checking your electrical grounds. If the AFM is faulty or has a bad ground connection, that might explain the rich running and the early overboost cut off. Bad grounds are well known to cause weird problems.

The main electrical grounds you need to worry about are on the bell housing behind the intake. If you do a search you'll find tons of pictures and explanations of how to clean them.

The other thing that springs to mind is: have you tested the fuel pressure regulator? They do go bad and usually cause a rich condition. I don't think that would explain the cut off though.

Anyway details on how to test this stuff can be found at http://clarks-garage.com/.

Good luck!

Dan Martinic

Modifying (raising) boost level without addressing other parts of the system (fuel delivery) is a recipie for problems. Immediately remove the DME/ECU and check for a replacement chip; if you don't have one, that's likely your problem.

As John explains, the stock system might see 15psi as a problem. Checking the grounds is part of the steps of the DME/KLR Test Plan (and in Clark's excellent Garage)

Lots to do... ...and I suggest diy: at this point, paying Cheech to learn more about your car could be money used towards buying the parts needed to improve your situation