Stephencs601

Jon,

I have a set of 72lb injectors I am going to put in my car in a few weeks. You can borrow them before spending the cash for no reason.

Techno Duck

Eliminated a few other components from the list today. Took out the injector resistors and also put in a different DME / KLR. Still no luck.

A BIG thanks to Brian Broderick for sending me not one, but two sets of DME / KLR's.

Honestly i think i have ruled out pretty much every component on the car. The only things left are the injectors, injector harness or some other fault in the engine harness somewhere. I am still too lazy to install the injector harness, so i am just going to hold off until the new set of injectors gets here tuesday before wasting anymore time on this car.

Steve, sent yah a PM.

Tom M'Guinn

You would think if it's the injectors it wouldn't be limited to that one specific rpm range. I once searched for over a month to figure out a somewhat similar driveability problem, and it turned out to be a slit in the bottom of the i/c coupler. I had actually taken it off the car and looked at it and pressure tested it and missed it every time. Hot couplers are more flexible too. Try this test: once warmed up, take a drive and floor it up to about 4000 -- let it hit full boost -- then stab the clutch and left off the throttle quickly. Do the rpms drop and hold steady at idle, or do the rpms drop low and then come back up to a normal idle?

Techno Duck

From my driving over the last 2 weeks or so, the problem seems to be more load related than RPM. I have noticed it does not always start at the same point, usually a variance of about 100-200rpm. The brake boosting tests were inconclusive, i swear it seemed like sometimes it would do it and other times it wouldnt. The severity of how lean it runs also seems to change with how much throttle i am giving the car. If i lightly accelerate (basically just cruising with barely any throttle) the AFR will only go up to about 15.2 or so, however if i am accelerating normally (just keeping up with traffic) it will go into the 15.6-16 range at some points.

The injectors i agree sound unlikely, my best guess is that when they get hot, at a specific load one of them may not be firing correctly. Pretty much every component in the engine bay is new or i have tested and can say with reasonable confidence can be ruled out. The last thing really left are the injectors.

I think i have ruled out a coupler leak also, i pressure tested to 17psi on an old VDO boost gauge and noticed no leaks (engine had been shut off maybe 10-15 minutes), even had the nose panel off to check the front mount couplers. Pressure dropped off at a fairly steady rate also.. nothing that would indicate a leak i think.

I will give your test a shot and see what happens, probably tomorrow if i have time. Doing that sort of thing on occasion (but to higher RPM) has caused the car to stall, i attribute it to the lightweight flywheel though.

Not sure what i will do if its not the injectors and i cant figure it out. I think either a LS1 swap or start searching for a 6-GT3. I spent about 3 weeks rebuilding the engine last time i had off from work, now i just spent another 3 weeks almost tracking down this problem, replacing a faulty thermostat and wiring in the piggyback. Ive missed 2 track days this year already ($500 down the drain) and wasted who knows how many beautiful days out under the hood of this thing when i could have been doing other things with my time off. Ive dropped an absurd amount of money on the car this year and have driven it about 500 miles, probably half of that was spent diagnosing this problem. Needless to say i am pretty much ready to throw in the towel.. id rather be enjoying my time off rather than getting pissed at this car.

Tom M'Guinn

I suggested 4k because it gives the motor less time to compensate for the over-metered air. If you take it to redline, and it still stalls when returning to idle, then I would for sure take yet another look for leaks (not trying to irritate, but sometimes they're tough to find). I've never used a light flywheel -- do other cars with lightened flywheels stall when coming back to idle?

Techno Duck

The stalling thing i only noticed after i installed the lightweight flywheel.. Infact the first time it happened i thought i blew the motor (and i distinctly remember not caring a whole lot!). I believe this is fairly common because the RPM's drop faster than what the ECU compensates for. Whether or not i had the idle set screw set correctly at the time is anyone's guess also.. .

Bri Bro

I am betting you did but...did you bypass the SMT8?

Techno Duck

Yup, one of the first things done was bypass the SMT8. Also switched back to the original MAF only map on the chipboard, same map i had been using for the past 3 years. I am fairly certain this started happening prior to the rebuild i did over the winter. I cannot say for sure but i am pretty sure i noticed it while driving the car down to my buddies place.

Techno Duck

Well thanks to Steve i was able to test out the injector theory. Swapped in his basically brand new 72lb Siemen's (same as mine) and the problem is still present. It was a total buzz kill because things were looking up on the 20+ mile drive i took where the problem did not come up once. It was relatively cool out and little traffic though.. once i parked the car for about 15 minutes the lean spot came right back.

If i have some free time tomorrow morning i am going to install the LR injector harness.. its literally the last thing i can think of.

I rigged up an extremely ghetto way of monitoring fuel pressure and was able to get one video of the gauge but its inconclusive... i do see a fuel pressure drop but i think its normal, just the vacuum dropping a bit as i accelerate and load comes off the motor. Will retest tomorrow.

The 996 GT3 is calling..

Techno Duck

Stuck my stock intercooler back in this morning (surprisingly easy) just to rule out the possibility of a pinhole leak in the front mount. No change.

During this time i also removed every coupler and IC pipe and inspected everything visually very carefully, everything looked perfect. Intake manifold gaskets are not blown out and the bolts are all tight. Dont think there are any problems here as i pressure tested it and found nothing.

Also finally installed the new injector harness, no change also.

Totally out of ideas!

refresh951

What about going to FQS Position 1, see if adding 3% fuel changes anything.

gruhsy

Change your fuel pump and see if it still does it. The pump may have a dead spot in it or it sticks at a certain point????

Run some ground jumper wires from various points to see if that changes anything???

good luck, if I can think of anything else I will post

Jason

Techno Duck

I have considered changing the fuel pump just because it is still original.. however someone correct me if i am wrong, but i am fairly certain the fuel pump is constant rate and for the most part should always deliver the same amount of fuel. I cant imagine why it would 'hiccup' under a relatively low load situation and only at a VERY narrow engine speed..while operate fine under medium load (under boost). I would also expect to see a drop in fuel pressure when it goes lean but based on the video i took i saw nothing that indicates a drop in pressure at the regulator. I am going to retry recording the fuel pressure gauge later so it can be ruled out for sure.. right now id say i am 90% sure it is not a fuel pressure related issue.

As of now the only 2 things really in question are the fuel pump and the Aeromotive fuel pressure regulator (less than 400 miles on it now). Perhaps the FPR is bad, but i would expect a fuel pressure change on the gauge. Also ive said it a few times now, but i am fairly certain this happened when i drove the car down to my friends place prior to rebuilding it (and installing the Aeromotive FPR).

I think it happens to be a coincidence the lean spot happens at 3-3200rpm, i personally think it is load related.. just all the conditions are right at that particular engine speed causing it to go lean. It will go lean almost all the time when accelerating in 1st and 2nd gear normally (just keeping up with traffic). When i get into 3rd gear, usually i am starting to ease up on the throttle as i am coming up to cruising speed.. occasionally (Depending how much throttle i am giving it) the lean spot will not happen. In 1st and 2nd the amount it goes lean will vary based on how much throttle i am giving the car.. more throttle the leaner it goes.

Tomorrow i am going to swing by a local indy P-car shop and see if they would be willing to take a look at it. I know they are really busy now but maybe i can leave it with them while i am away. I always goto them for alignments..etc, not sure if they will even want to bother doing any engine related work considering how heavily modified the car is. Plus they are used to the track only, lambo, ferrari, exotic classes of cars.. not my stupid pos 25 year old 944T... .

Techno Duck

So after putting a few more miles on the car i can safely say the car will go leaner depending how much hard i am accelerating. What is truly strange is going over the log file spread sheets i honestly see nothing that would indicate there is even a problem. So i think its safe to say nothing that the piggyback is datalogging is causing the problem (MAF + wiring, TPS, piggyback itself).

Here is a video of the fuel pressure gauge. The hookup was so ghetto i didnt even take a picture of it.. . But basically consisted of my Hero Cam zip tied to the strut tower with a flash light under the hood.

In the video i am doing 1st through 3rd gear shifting at just under 4k rpm. So i am going well beyond the 'lean spot'. The lean spot happens every time in the video also. Is the oscillation i see from the gauge look normal (30 second mark, 40 second and 49 second marks it starts)? The gauge is not dampened so i figured some bouncing would be normal but this seems a bit excessive. Ive honestly never really looked at a fuel pressure gauge while driving so i am not sure what it should be doing.

BTW, this video is no boost at all in 1st and 2nd.. getting up to maybe 10" of vacuum at most. Think i hit boost just a little in 3rd towards the end.

Van

Just for sh-ts and giggles, have you tested the vacuum system for leaks? How about the accumulator that lives next to the battery? (there's a check valve, so it might have held under pressure)

Is your FPR vacuum line plumbed directly to the banjo bold on the intake, or through other lines?