Redneck Truck

I decided to spy on a few measuring blocks sitting at home after my car had been particularly troublesome today. With the A/C off and the engine at operating temperature, measured throttle position mimicked what my foot was doing almost exactly. As for the engine's behavior, it had almost nothing to do with actual (and measured) throttle position.

1. Very fast to rev up from idle. A few times, when I gave the pedal a 1/3 throttle tap, it would rev to 6500, and one time it even held there for about half a second.

2. Slow to acknowledge release of gas pedal. If I press clutch pedal in and release gas pedal simultaneously, engine continues to rev another 2-500rpm before dropping off.

3. Very slow to rev down to 2000 or so, and takes 2-3 seconds to come down from 2000 to idle. It does not behave like it has a lightweight flywheel and a closed throttle.

4. shaky when settling down to idle. I can only liken it to the bucking you get from stabbing or releasing the gas pedal when you've got broken driveline mounts, only the RPMs move that way in gear or not, only when settling from 1500 to 500.

5. 500rpm idle once settled. This seems a bit low, especially as adaptation channel 1 of 133 calls for something like 730.

6. Revs to 1800rpm when I blip the throttle, and holds until I add load (steer side to side, A/C compressor on). I have allowed it to idle at that speed for close to a minute.

The key metric in this discussion is ignition timing. At the 500rpm idle, it reads between 5°BTDC and 5°ATDC. When it's idling at 1800rpm, it's between 27°BTDC and 33°BTDC. When it's doing its shaky settle from 1500 to 500, it's all over the place, between 33BTDC and 5°ATDC. It's as if the computer is using ignition timing to influence RPM for god knows what reason, or it's unable to control ignition timing for god knows what reason. So what is the computer thinking when it's doing this, or what has failed that is allowing this to happen?

2.7Lturbo

have you looked at the plugs? checked for vaccum leaks? loged FATS?

Redneck Truck

Replaced intake manifold, throttle body, suction jet pump, CCV, and EVAP purge valve, replaced vacuum lines up front, deleted kombivalves and vacuum control valve, applied vacuum to all other fittings, no leaks that I am aware of. So, if there are vacuum leaks, I don't know where they could be.

Have not checked spark plugs.

Unfamiliar with FATS test, Googled it, 4200-6500 in 3rd gear? Which measuring blocks?

On my old Nissan pickup, I stabbed the distributor one tooth off in each direction and the idle changed even though airflow was presumed the same.

With it a tooth on the retarded side (10ATDC base), it idled at 700 perfectly quietly and the exhaust manifold glowed under load. You could take it down to less than 100rpm without it stalling.

With it a tooth advanced (30BTDC), it idled at close to 3000rpm and revved like a sport bike.

socalclimber

Redneck,

Did you ever figure anything out on fuel pressure regulator?

Does FPR hold vacuum (actuate) when testing with vacuum pump? You said you tasted fuel when you applied your own vacuum which may indicate a blown FPR.

One factor of fuel metering is engine load (manifold vacuum) via the FPR, i.e. lower fuel pressure at idle (high vacuum) and higher fuel pressure at load (low vacuum).

These are the test specs from Bentley:

2.7L and all V8 idling, gauge connected inline bet. fuel rail and fuel suply line.
-Fuel pressure regulator vacuum line connected = 3.5 bar (51 psi)
-Fuel pressure regulator vacuum line disconnected = 4.0 bar (58 psi)

Residual pressure, leave gauge conencted, shut off engine, wait 10 mins
-Warm engine = 3.0 bar (44 psi)
-Cold engine = 2.5 bar (36 psi)

If your FPR is not doing its job, you MAY have high fuel pressure at both idle and under load, this MAY be affecting the driveability.

Just throwing this out there.

Redneck Truck

Really appreciate you finding that for me - I was a little curious, though I don't have a gauge to test with at this point. However, I replaced the regulator with a known good one from a mechanic buddy, and nothing changed, so I surmise that even if my regulator may have had a ruptured diaphragm, it still held vacuum and may have been doing its job well enough that my symptoms can be assumed to be elsewhere. However, if I get my hands on a fuel pressure gauge, I will definitely test it against those specs.

2.7Lturbo

ill check the fats log to fun, also get a picture of here of you spark plug, spark plug fouling can be a gold mine of information of whats going on in the engine.

did this start after deleting the kombi valves?

Redneck Truck

It started after I swapped to a manual transmission. If I've got the vacuum control valve for the kombivalves hooked up, the computer could certainly be expecting different o2 readings from pumping air into the exhaust, so I'll disconnect that vacuum valve and see if the symptoms change. I was under the impression that the kombivalves were only activated during warmup anyway, though.

When I pulled the spark plugs 10k ago, they were all uniformly gapped, clean, and in perfect condition, but I will pop them out again just in case.

2.7Lturbo

just pop one out, and while the kombivalves are ostensibly only for warm up i wouldnt be surprised if they did something at idle as well

Redneck Truck

With that logic, the computer would see the missing vacuum valve and know not to expect airflow. I'll give it a shot this afternoon.

Just Me

well, if over and under revving were the first symptoms, i would begin with components that directly influence them:

1. MAF
2. EPC

Specifically i would imagine a throttle (motor controlled, byt eh epc) sticking in the open position, or maybe sticking int he closed position, then, jumping when it clears the (imagined) carbon or dirt ridge that the butterfly valve creates as it sweeps open and closed 10s of thousands of times...... BTDT!

Do you have codes?

next, i would imagine, since it sounds nearly impossible, things like power supply fluxuation at the computer, causing complete havoc. I mean, what else explains this, aside from gremlins, sunspots and Microsoft?

Juts my two cents

G