Van

p.s. a fuel pressure regulator is easy enough to change... can you put in a factory one, or a 3 bar one and compare it's performance?

I seem to recall a larger fluctuation on my fuel pressure gauge (mine's cockpit mounted) but I'm running stock injectors with the stock FPR. I mean fluctuation from idle to WOT (vacuum to no vacuum).

Techno Duck

Putting the stock 3bar regulator back on is something id like to do to rule out the FPR. But i would need to put the stock rail back on also. Easy enough but i need to find some adapter fittings to fit the AN hardline fittings i have. I actually figured out the sizes but think i threw out that damn paper i wrote it down on..

The regulator is being run with a dedicated line off one of Laust's vac manifolds.

This will have to wait until i get home in August.

Wormhole

Hey Jon, the fuel pumps I've seen and had go bad intermittently start with a low rpm miss. Once the fuel volume and vacuum pressure to the regulator increase it’s less noticeable until they are total dead.
Good luck

Ronin-951

Much as I would like to see you dump your car for low dollar, [to me] there's a trick used in electronics troubleshooting you should try. A refrigerant spray to cool suspect components one at a time, ie, capacitors are notorious for intermittent, narrow range problems. Since you have already eliminated the DME/KLR, I'm going out on a counter intuitive limb by suggesting the coil might be the lean condition culprit - part of a LCR circuit that might be working TOO well at a specific frequency/rpm. Since this should cost you less then $10 to try, I can live with myself for the insane, crazy, stupid suggestion.

Techno Duck

Coil was replaced with a new Bosch last year. I havent heard the term LCR circuit since i took EE3 a few years ago.. that instantly gave me a headache trying to remember what it was .

I am going to try replacing the fuel pump when i get home. I would have done it already just because of its age but i am getting to the point where i am tired of just throwing money at the car. $170 for the 'high volume' pump (looks like its just an old Merc 450 pump) and while i am at it replacing the other fuel lines isnt so bad. Besides, a pump that flows more will be useful when i give up and decide to put in an LS1..

When and if i ever figure this out i will literally have a brand new car! I think the only original part that will be left is the fuel vapor purge diaphragm.

Anyone have any comments on the fuel pressure video? Just wondering if that amount of bouncing around from the gauge is normal.

TurboTommy

Techno Duck;
I feel your pain, man. It must be very frustrating.
When you witness the lean spot, is there also a corresponding performance hiccup? Another words, do you feel a hesitation or loss of acceleration between 3 - 3,200 RPM. Or, is it that you just see it on the wideband?
Also, it's not clear in this thread (maybe I missed it) if it's also lean on full throttle at this RPM?
You say it is load dependant; but, if it's always happening at the same RPM, wouldn't it mean it's RPM dependent? Load just varies the severity?
When underhood temps bring about this problem it's gotta be 9 times out of 10 excess resistance in electrical components, you would think.
I wonder if doing a continuity test (after the engine has been running for awhile) of all the suspect wires at the DME plug might expose something.

Techno Duck

There is a noticeable drop in acceleration when the lean spot occurs... not a huge drop but noticeable.

I have not tried going full throttle yet, with the lean spot, not to mention a very rough tune (meaning not dialed in) i do not have much confidence to go WOT with it yet.

Yes, the amount of load on the car increases the severity. Looking at the wideband even when lightly accelerating i can see the AFR go slightly lean.. it will go from say 14.3ish to 14.9 momentarily during the lean spot even when the car is not completely heat soaked. So when the car is hot the problem seems to get worse.

I managed to get a few more days off before needing to head back offshore so tomorrow i am going to replace the fuel pump. This will essentially rule out every sensor / electrical related item on the car.. with the last thing left being something mechanically wrong (leak somewhere) or it being an issue in the engine wire harness (hopefully not!).

I just cant see there being a problem in the engine wire harness. I am logging the DME temp sensor, MAF and TPS. None of which show change when the car goes through that lean spot. This to me says everything is working as it should..right?

As per Van's suggestion, i am going to take a close look at the vacuum reservoir by the battery tomorrow.

That leaves me with these things..

1) fuel pump dying.
2) fuel pressure regulator bad?
3) some kind of mechanical problem.. leak in intake track i am still missing?
4) problem with engine wire harness

Just want to say i appreciate everyone's input, including Tom and Brian who shot me some more ideas through PM over the last few days. The good community is one of the reasons i love the 944 series cars so much.

Stephencs601

Jon,
You deserve to have this car working before you leave. We will all pray to the car gods that this gets fixed today.

Tom M'Guinn

Have you tried clocking the MAF differently?

fast951

Jon, how are you logging the TPS? I emailed you already, not sure if you are getting the emails.. Have you checked the Idle and Full load signals at the DME, when it's fine and comparing the results to when it acts up?

Tom, MAF signal is steady and not showing any sign of irregularities. Heat soak is causing a sensor and/or a wire/connection to act up.

Techno Duck

Fuel pump changed; still getting the lean spot. Honestly i didnt think this was going to do anything so i am not all that bummed out.. but thats not to say i am not pissed i cant figure this out! On the bright side i now have a Bosch 'HV' pump in so i am set for when i finally decide to just put in a LS1.

I thought up a way to get a better idea of what the fuel pressure is doing when it goes lean, going to test tomorrow. I am doing this so i can be absolutely sure the Aeromotive FPR is working correctly. I am still wondering if the bouncing needle on the fuel pressure gauge in my video is normal or not.

This is how i tested fuel pressure the last time i was having some problems and using the stock rail. This time i should have enough hose to get the gauge inside the cabin and a buddy that can watch it for me.. .

DVC

Sorry to hear this, Jon. I hope you get it solved ASAP.

Techno Duck

Well today was very frustrating and a total bust. Heavy rain and an absurd amount of mosquitoes so was not able to do much.

I emailed Aeromotive asking them about the fuel pressure and they basically said they are doubtful the regulator is the problem, but the bouncing around fuel pressure does not look like the norm. They told me this type of thing typically happens from excess air in the fuel. Typically caused by a restricted suction or an obstruction after the pump. I ordered all new hoses (pump suction and discharge), the tank screen and a new fuel filter yesterday.. when i get home again i am going to replace those items so everything is good for another 25 years.

I am going to investigate it further tomorrow, ive got my buddies fuel pressure gauge ready to go on so i can see if the fluttering pressure happens at the same point as the lean spot while accelerating. Heading to ALMS @ Lime Rock tomorrow, so hopefully sunday morning i will be able to check. Until then.. time for some much needed booze

Techno Duck

Was able to do the fuel pressure test today though the results were inconclusive. I was using my buddies Snap-On fuel pressure gauge and did not see the same oscillation from the gauge that i saw from the Aeromotive gauge. Its possible the Snap-On gauge has an internal damper or simply does not react as quickly as that Aeromotive gauge so i cant be for certain of what was happening. The gauge is pretty old so not sure it can be trusted.

Putting the car into storage tomorrow morning. When i get back in August i am going to hook up my stock fuel rail. Another thing i may try is getting a longer jumper line so i can route the Aeromotive fuel pressure gauge temporarily into the cabin so i can see whats happening.

I drove around for a while with the car last night and noticed some more things. I drove around for nearly 30 minutes and never had the problem start. Stopped a few times to feel the FPR and it was fairly warm, though i could comfortably keep my hand on it. Stopped for about an hour and let the car sit. Came back, felt the FPR and i could barely keep my hand on it for more than a few seconds. Started the car and the lean spot was very noticeable, say 15.6-15.8 AFR. Drove about two miles down the road and i noticed the lean spot was not so bad.. maybe 15.2-15.5 AFR. Stopped and felt the FPR, still hot, but not as much as before. Basically i am thinking when the FPR heats up it starts acting up. Of course this could just as easily mean something else is heat soaking, but not sure what else is left under the hood..

-DME temp sender new, wiring connector and wires replaced / repaired. Datalogging also shows nothing abormal. Plus i would expect the car to go rich if this were the issue.
-TPS. New switch put in, tested with my old and the problem still happens. Wiring connector and wires replaced. Seems unlikely at this point it is TPS related as i would expect the car to stumble at various RPM ranges based on throttle input. But the fact i can hold the TPS steady and accelerate through the lean spot leads me to believe this is not the issue
-Fuel injectors. Tried 3 different sets, my originals, Steve's barely used set and the new set i bought is in the car now
-MAF. MAF signal is being data logged and shows nothing that would indicate a problem here.
-ISV was replaced with a new one with no change
-O2 sensor is new, When disconnected, during the lean spot the car will go leaner. I take this as the sensor is working, or atleast doing something.

That really only leaves the FPR left that i think could cause what i am experiencing.

After being at ALMS @ Lime Rock yesterday and hearing the C6R going around the track..it made me really tempted to put in a V8.. .

Tom M'Guinn

But why would a FPR cause the motor to go lean in one specific rpm range?