Fuel Pressure Dropping and Engine Break-up at 3500 RPM Regardless of Load/Boost

[Chrisco]

Scenario:

-Vehicle: 1992 N/A Supra
-Engine/Harness/ECU: 1991 7MGTE swap
-Turbo: Modified CT-26
-Air Flow Metering Device: MAFT (not PRO) w/ 3.5" GM MAF
-Fuel: Aeromotive AFPR, Walbro 255lph, Precision 550cc injectors, Driftmotion Fuel Damper Bypass and AFPR kits installed. Fuel pressure was set to 40psi base with Fp and +B jumped.
-Ignition: NGK BKR6E Spark Plugs gapped to .028" Issue is present with both Stock Ignition coils and wires, as well as 2JZ VVTi Coils and wires (Tookwik mod). Igniter ground mod has been performed.
-Timing: Set at 10* following TSRM. The CPS wiring is all good. I replaced the harness side connector and checked the wires/connector on the CPS and they are good as well.
-Knock sensors rewired with RG-6 coax cable. Sensors and wire have been tested and are good.
-No Codes are Present.



Issue:

The car drives GREAT in all throttle/load conditions, in and out of boost, with two exceptions:

*It breaks up terribly ONLY between 3450 and 3550 RPM. It does this whether there is no load (free revving), or load applied; as well as in and out of boost. The only difference between low load/no boost and greater load/in boost is that when there is little-to-no load applied the engine breaks up bad enough that it won't carry over past that ~3500 RPM threshold. Conversely, while under load/boost/accelerating at a greater rate, it momentarily breaks up at that point, but is hardly noticable and is able to pass the threshold due to momentum of the engine with the car accelerating in gear. I have not yet wired my wideband O2, so I haven't determined if it's a lean or rich condition; however, due to the sound (sounds like it's on a two step when free revving to 3500) and consistency of occurrence, I believe it is some sort of computer controlled ignition issue.

**Secondly, the warm idle is far too low at approximately 400-450 RPM. Because it idles so low, the car will die out nearly every time the clutch is depressed and the revs are allowed to drop without modulation of the throttle. If I'm careful with the throttle, I can get it to hold an idle, but it is at that very low point.


Diagnostics Performed:

I calibrated and tested the TPS according to the TSRM and JSeaman's writeup (http://www.supramania.com/forums/sho...osition+sensor), and everything was in spec with the exception of one reading. Resistance measured between E2 and EDL are supposedto be 2.3k ohms with a 50mm feeler gauge between the throttle and set screw; my reading was ~12 ohms.

I've been reading VF (Vf1 > volt meter > ground) and while adjustments with the MAFT create physical/audible changes in the way the engine runs, the VF signal seems to be unaffected. From my observations, the VF signal has five readings: 0.0xx volts, 1.0xx volts, 2.2xx volts, 3.5xx volts, and 4.9xx volts. The readings vary across this range, most commonly when either under throttle or lifting completely (goes from 2.2-4.9 while under throttle to 0.0xx when completely off the throttle). At idle, the reading has been any one of those five. Again, adjusting the Idle dial on the MAFT seems to change the way the engine runs, however the adjustments don't seem to have any direct effect on the Vf readings.

I'll update the thread as I progress through my diagnostics, but any and all help would be greatly appreciated.

-Chris

 

[Chrisco]

Transfered from my previous thread (separating them since this is a different issue and the previous two were resolved):

I haven't heard of it happening on the 7M, but on the 1JZ removing the fuel-dampener (which I see you have removed) causes this type of problem. The dampener de-Q's the fuel rail, without it it can resonate at certain frequencies resulting in large pressure variations along its length. Try putting it back on if you still have it.

Hrmm... I hate to do that, but I believe I still have a dampener so if some of my further diagnostics don't sort it out then I'll go ahead and try it.

Do you think that such a harmonic would be seen via a fuel pressure gauge? Perhaps a fluttering of the needle because of the pulses being produced? I'll try taking a look next time I get outside.

 

[radiod]

Did you double check the timing after calibrating the TPS? That can throw the timing off if things changed.

 

[MkIII FTW]

Are you noticing any fuel pressure drops?

 

[T3rril79]

Does the engine tend to have this issue only when it is under a load and you are driving or does it do it as well under a free rev while in neutral (parked) as well. If so see if you have any major changes in the fuel pressure at that rpm.

 

[Chrisco]

Update:

Ok, so after some further investigation/driving of the vehicle, I've found that it the range where it breaks up varies due to a much greater issue that has just been discovered: fuel pressure will NOT stay set where I have set it with the AFPR. I have found that if I adjust the fuel pressure, car on or off, and go for a ~5 minute drive, the static fuel pressure starts to drop. I can't watch it drop, but it drops after driving, and rises after setting over night. For instance, last night I drove the car enough to get it good and hot. I turn the KOEO (Key ON, Engine OFF), jumper +B to FP and set fuel pressure to 40psi. I then took the key out of the ignition, locked up the garage, and went to bed. This morning before work I go out to the car and, before starting it, turn the KOEO and jumper +B to FP and check fuel pressure. It's at 50psi! I adjust it back to 40psi and leave work work. I get to work and the car will barely idle so I pop the hood, get out, and fuel pressure is now bouncing between 0-3psi!! Now, I had experienced 10-15psi drops in pressure (from 40psi down to ~25psi) over the last couple days, but I had attributed that to what I thought was a bad cold start injector. At first I had just disconnected the connector, but after the issue persisted I went ahead and removed the banjo bolt and installed a regular bolt (1/32" shorter than the banjo) to completely block it off from flow. I now know for sure the CSI has nothing to do with fuel pressure dropping.

Now, to my knowledge, I only have three (well, technically eight if you count each injector separately) points in the fuel system where I can be losing pressure, barring an external leak; the fuel pump, injectors, and pressure regulator. Lucky for me, a co-worker had a spare AFPR sitting on his desk at work. I swapped out the AFPR's over lunch to find the following result:

After swapping the AFPRs, I turn the KOEO jumper +B to FP and set fuel pressure to 40psi. Start the car, pinch off vacuum source to the AFPR, and verify it is still set at 40psi... all is good. I then drive about 5 miles to where I'm going for lunch, park, pop the hood (car still running), pinch off the vacuum source to the AFPR, and fuel pressure is now 3 psi lower at 37psi. Rather than making any adjustments, I go in and eat my lunch. Come out 45 minutes later, start the car, pinch off the vacuum source to the AFPR, and fuel pressure is now at 35psi... WTF? So the issue is not the regulator.

Here's the final kicker. There is an EXTREMELY intense pulsation going on inside the pressure lines of the fuel system. It is so profound that it sounds like a knocking, similar to an exhaust leak, inside the car. The pulsation can be felt when touching all of the feed side fuel lines from the pump all the way to the pressure regulator. The pulsation is felt in the return line, but is very faint compared to the feed lines and rail. Also, the frequency of the pulsation is directly related to engine RPM; as RPM increases, the rate of the pulsation increases with it. As RPM falls, the pulsation frequency slows as well.

So, now as far as I can tell, the issue is narrowed down to either the injectors or fuel pump and could possibly be an electrical or mechanical related failure. However, I highly doubt the issue is injector related because if I was losing 40psi through a leaking injector, my oil would have a couple gallons of fuel in it by now, and the exhaust would be pouring out unburned hydrocarbons. What I don't understand, though, is why the pulsation would only be present with the engine running.

All of this now being said, I can definitely attribute the 3500 rpm break-up to a lack of fuel pressure. After adjusting fuel pressure and immediately taking the car for a test drive, there is no hesitation or break-up at all whatsoever. As the car continues running, fuel pressure begins to drop and the break-up returns. A friend of mine has a scope so I plan to use it to check the fuel pump electrically first. Depending on what we find, I may also try running completely seperate B+ and ground wires to the pump to isolate it from the factory electrical circuit.

If anyone has some thoughts on this, I'm all ears!

Are you noticing any fuel pressure drops?

In short, yes.

Does the engine tend to have this issue only when it is under a load and you are driving or does it do it as well under a free rev while in neutral (parked) as well. If so see if you have any major changes in the fuel pressure at that rpm.

Please refer back to the initial post.

 

[T3rril79]

OK that is making since. My car breaks up at 3500 on the head. Even with my new AFPR I am still seeing 3-5 psi fluctuation in the fuel pretty at idle. Even more if I rev it. Upon further investigation, I noticed that my car will not hold the pressure in the line after it's been running. My walbro has been making a curiously obnoxious whining noise that is extremely loud. I did some research and this has been dubbed "the death whine". I'm going to buy a new Denso MK4 pump and see if my hunch of a dying fuel pump is correct.

 

[90mkiiisupra400hp]

Have you figured this out?

 

[Chrisco]

Have you figured this out?

I apologize for taking so long to follow up on the thread; life has been very hectic as of late and I hadn't worked on the car in over a month.

This past weekend I went back out to do a little bit of maintenance and found that the car would not start. Long story short, it was due to corroded connections on the pump hangar inside of the fuel tank causing an open circuit and the pump to not run. I completely replaced the wires and connections inside the tank and the car started right up. I thought that the corroded wires may have been contributing to the fuel pressure drop off, but that problem is still present.

I also went ahead and ran an isolated B+/ground circuit from the battery, to the pump, controlled by a relay via the factory fuel pump wiring. So, it now operates the same as from the factory, however it has constant battery voltage rather than the modulated voltage through the fuel pump resistor. Again, this had no effect on the pressure drop.

I have not yet checked the pump with a scope, but that is the last test I have to do before I go ahead and replace it. I'm not sure when I'll get around to doing that, but I'll post back as soon as I do and have some results.

-Chris

 

[3p141592654]

A fuel pressure dampener should quell the pulsing. As each injector turns off, it basically creates a"water hammer" effect same as you get when a washing machine turns off. All these pulses can add up to make some nasty resonances at certain rpms. For the pressure variations, I would start by making sure your pressure gauge is repeatable, especially over temperature.

 

[Chrisco]

A fuel pressure dampener should quell the pulsing. As each injector turns off, it basically creates a"water hammer" effect same as you get when a washing machine turns off. All these pulses can add up to make some nasty resonances at certain rpms. For the pressure variations, I would start by making sure your pressure gauge is repeatable, especially over temperature.

Thanks for the confirmation on the pulsation. Has anyone else out there with a 7M experienced the same pulsation/knocking after removing their Fuel Damper and installing the bypass kit from Driftmotion?

As for the pressure changes, I've tried two different AFPRs with two different gauges with the same results; so, while unhappy with them, I'm confident in the values I've seen so far.

 

[supraguy@aol]

I think 3P is right, because if the fuel rail "water hammer" does happen, then *maybe* the resonant frequency happens to be ~3500rpm, given your injector size, fuel pressure, and all other variables.