RET

I have a new problem, or at least a new symptom of an older problem: after installing a newly rebuilt MAF, the car will not idle and the engine will stall. Unfortunately, another MAF (with unknown provenance) borrowed from Bill Ball seems to work just fine, and my original MAF did not cause any idle issues at all.

It would seem at first that the “new” MAF was faulty, but when it was installed in Bill's car, the engine ran fine. Even worse, when we installed the MAF from his car in mine, the engine would not idle – the symptoms matched those using the rebuilt MAF.

The MAF incompatibility does not make sense: I of course have no idea what this means, Bill is puzzled, and Louie Ott could offer no definite advice. I hope that someone here can help....


Recap of the earlier problem and steps taken to date: the original MAF allowed the engine to start and idle well but it had no power until warmed up (and it still idled well...); there was also intermittent and momentary hesitations (very brief losses of power), at freeway speeds.

Bill loaned me a spare MAF. I swapped it for mine, and the problems went away; I reinstalled mine, and the problems returned. Swapping in his LH and EZK units had made no difference at all, nor did replacing the FP, LH, EZK, and X-bus relays, as well as theTemp2 sensor. I also checked fuel pressure and the various sensor signals at the EZK and LH connectors.

The old MAF was sent to Louie Ott. He called to tell me that it was not too bad – it should cause somewhat lean running and sent me a newly rebuilt unit.

I disconnected the battery, replaced the loaner unit with the rebuilt MAF, reconnected the battery and started the engine. Engine started quickly, and ran smoothly if the throttle was held, but if the throttle pedal was released, it would not idle. I was not too worried then because I thought that the LH just needed to sort itself out, so I drove the car over the next few days. It made no difference: the engine would not idle.

Thanks!

Bob

Speedtoys

Time to investigate the continuity of the plugs at the MAF end, and the ECU end as you wiggle the connector.

Mrmerlin

how many miles?
has the intake refresh ever been completed when?

jeff spahn

I was going to say check the connector and the wiring back to ECU but got beat out.

RET

Thanks for the replies.

FWIW - All contacts in the ECU and MAF connectors looked good (not deformed, corroded, etc.); the intake refresh is a couple of years old, with new ISV, FPR, fuel injectors, etc. (even a new O2 sensor), the dampers are old but have been inspected for leaks during this troubleshooting cycle.

I will test the wire harness for continuity and internal shorts in the next few days, but if I fail to find anything, I will replace the O2 sensor.

If none of the above makes any difference, I will try lowering the fuel pressure: the car has 30 lb injectors; my old MAF would, per Louie Ott, caused it to run pretty lean; maybe Bill's spare MAF (of unknown provenance) also causes lean running and the newly rebuilt MAF as well as the known-good MAF (rebuilt by Louie in '14) from Bill's car might be causing a very rich mixture at idle with my oversize injectors..... and maybe I am trying to over think this....

Bob

FredR

How are you running 30lb injectors- are you custom tuning with ST2 or was it custom tuned by Marc Thomas when he built the motor back in the early Sharktuner mark 1 days?

Is your 928 the one that had about 250k miles on the clock but the motor about 80k miles on the build when you purchased it?

Rgds

Fred

worf928

Has the engine speed (aka. crank position, impulse sender) been replaced in the last 6 - 10 years?

The usual, most-probable, failure mode of the speed sensor results in a no-start. However, there is an unusual less-probable failure mode that results in the EZK computing the wrong speed value from the signal. This results in very poor running below ~1500 rpm.

Given that you've dealt with all of the common, probable failure modes...

worf928

Oh! Yeah: Good question Fred. OP's symptoms and MAF anecdotes are consistent with over-fueling at idle. I missed the 30# injector thing.

But, on the other hand, I'd be somewhat surprised that a stock 928 with no custom tune would even start on 30# injectors.

On the gripping hand: no-tune for 30#-ers would result in a brain-swap having the described (non-)effect.

worf928

Do the symptoms occur during the first 2 - 3 minutes after a cold start? If so, then a new 02 sensor will have no effect. If yes, then a new 02 sensor might help, but won't cure the problem.

The 02 sensor is used for fuel trim once the engine is not shop-cold (2 - 3 minutes). Your problem goes beyond fuel trim.

RET

Hello Fred,

The engine was built and tuned by Devek, to include the 30 Lb injectors... I assume that they were properly accounted for in the process, but I have no idea of any of the details - for instance, was the MAF in specification, and were the idle fuel maps changed?

FWIW, the car had over 100K miles when purchased by me, but as I recall the engine was represented as having only about 20K...

Bob

jcorenman

Cool car!! But how was it tuned? Modified chips, or adjustable fuel pressure, or ??

I agree with Dave, this sounds very much the new MAF has put the LH out of its adaptation range and it is running rich. Old MAF's run lean, if it was tuned with an old out-of-spec MAF then switching to a new one can easily make it too rich-- depending on how carefully the tuning was done. The LH can increase/decrease fueling about 20% (the stored adaptation).

Before getting too excited, disconnect the battery for a moment to reset the LH, then drive the car for a while to let it learn about the new MAF, and try a cold start and see if it behaves any better. For a custom tune (be it Sharktuner or fiddling with injectors and fuel pressure) it is very easy to adjust the tuning with an old, aged MAF, and then swap in a new MAF and have it fall on its face because the LH adaptation is now out of range.

If you have a wbo2 (before any cats), then warm up the car, disconnect the O2 sensor (to disable the LH adjustment), disconnect the battery momentarily to reset the LH, then fire it up and check the AFR. Adjust fuel-pressure or whatever you've got, to set AFR around 14.7-ish. (Do this quick before LH starts adapting again, stop and reset the LH as needed).

FredR

Hi Bob,

Your car sounds very similar to one that Devek offered me many years ago but your mileage profile does not fit in with that in my decrepit grey matter but no matter. As I remember the one I had in mind was originally built for a chap whose first name was Troy.

I am not sure how it would have been tuned by Marc as that would be time line dependent I suspect. Until the advent of Sharktuner such work was difficult to say the least. High output motors had issues because the airflow took the stock system out of range on the voltage signal to the LH wherein airflow read was pegged out. This became apparent to John when the first version sharktuner was developed and then the need for a different mass air-flow range was adopted thus the Super MAF. ST2 some years later was a major step forward in that in permitted home hacks like myself to play around with the tuning of their engines with a neat piece of kit that was within reach financially speaking.

For someone like yourself with that build spec you really should think about investing in a set of the ST2 kit. That alone would probably tell you what is going on with your motor at this moment in time. If by any chance you fry your memory modules you will have no way back for starters. Indeed I am sure that one of John's agents can recover the currently active bin files for you to flash directly into the PEMS that come with ST2 so you have a start point.

Rgds

Fred

John Speake

Devek did buy one of the original Mk1 Sharktuners. I'm not sure if they bought PEMs and they came out later, maybe after they gave up with 928s.

So it is likely that you have custom EPROMs in there, not PEMs.

I remember Marc telling me about the time he tuned a stroker which was spinning the wheels at 90mph.

RET

Hello All,

Many thanks for the responses.

While I hate to expose my ignorance about anything, I must admit that I have absolutely no idea how this engine was tuned.

FWIW, the LH has an EPROM and the engine does have an AFPR, but the AFPR that it came with had a vacuum leak and I replaced it with a new one without considering whether the pressure may have been set differently from stock, so I set the new one to stock pressure. I did not notice any change whatsoever with the change.

I followed Louie's instructions when the new MAF was installed: the battery was disconnected. When the car was initially started after the installation, it instantly started but would not idle above ~600 RPM, and this idle speed got lower as the engine warmed until it stalled about the time the temperature gauge showed signs of life. It would restart with throttle applied and did its best to try to keep running, but the idle was low and uneven, and of course, it would eventually stall.

I had thought that the LH needed to adapt and drove it around for the next day or so. It ran great otherwise, but would not idle after it was warm.

Fred - some history... yes, this was Troy's car, and while I am certain that I do not remember ever having forgotten anything, my exact recollection of the details seems a little hazy.... I bought the car through Devek (they had possession of the car and Susan handled the sale; I never met Troy) just as they were going out of business and the promised, detailed documentation on the car was never delivered to me and was lost during the confusion of their shutting down. IIRC, the odometer showed 130-140k (?), and the engine had been more-or-less recently gone through.

Shark Tuner - there is another oddity with this car: it has both the round diagnostic connector by the passenger door and the rectangular one on the side of the ECM frame, but the communication lines for the ECU only go to the rectangular port. I traced these wires out sometime ago, and have them documented somewhere,,,,

I will not be able to try anything for the next couple of days, but hopefully I will have sorted something out by SITP.

Bob

jcorenman

Bob, good info-- thanks. The 19-pin round diagnostic connector came with your '89 (part of the car harness). The rectangular 12-pin connector is part of an '87-88 engine harness, so that must have been swapped at some point in the past-- maybe the 6.5L came with an early harness. Either will work for Sharktuner, you just need the '87-88 12-pin cable.

One more thing for your checklist: Make sure the vacuum line is connected to the FPR (and has manifold vacuum), without that being connected the fuel pressure will be higher at idle (and cruise), making it richer (which is what this still sounds like).