Spark Advance Hangs at 25 Degrees at Hot Idle

[mbruffey]

97 EB 5.0. NO CODES.

Problem: Intermittently the engine will develop a slightly fast idle when warmed up, about 900-1000 RPM. Normally it hangs around 700. This is the classic symptom of a vacuum leak, and I think that this is possibly verified by the presence of tons and tons of "uncalled" misfires. But, perhaps low fuel pressure instead?

(Note: the TP sensor voltage fluctuates a little at idle but not enough to cause this. TPS does change the advance about .5 degrees up or down when it fluctuates, and remains between 17 and 20 degrees. I think that I will replace it but I really don't think that its the cause of the problem I'm after here.)

I can read a LOT of codes with the computer software with engine running. After scanning a number of outputs for a while, I noticed that the FUEL TANK PRESSURE was NEGATIVE 10 at the same time that the SPARK ADVANCE was "stuck" on 25. All this is at dead idle at 900+ RPM in park.

I decided to remove the GAS CAP. FUEL TANK PRESSURE goes positive (+0.6) and SPARK ADVANCE returns to 18 degrees. Put cap back on; fuel pressure goes negative again and advance back out to 25 degrees.

Software indicates that the FUEL TANK PRESSURE has something to do with EVAP/PURGE operations. I'm guessing that under certain conditions the system goes into an EVAP cycle (fuel tank was FULL at the time I'm reporting here). I had replaced the EVAP Solenoid under the battery compartment last summer, and did upper and lower intake gaskets too, to eliminate possible vacuum leaks. Gasket change improved idle, but not fix this issue. (Been chasing this one a long time.)

Guesses:
(1) Vacuum Leak when EVAP on
(2) Low Fuel Pressure = LEAN (but why advance go high then?)

Three other bits of information:
(a) Fuel Pump Secondary Monitor High = YES (NORMAL?)
(b) Fuel Pump Duty Cycle = 100% (NORMAL?)
(c) Case ground for the Fuel Pump Varies from NEG .07 to NEG .09 depending on RPMs. (more neg with accel)

Questions:
(1) Does the EVAP cycle normally kick the advance to 25 degrees? It is stuck exactly on 25 and does not fluctuate a bit.
(2) Does the EVAP cycle open a line that could be the cause of a vacuum leak?
(3) Should the tank "breathe" during EVAP to prevent negative pressure? Could the charcoal filter be full, or some line to/from it be clogged?

Thanks in "advance" for your kind input!

mbruffey

 

[koda2000]

As no one else has responded, I’ll give you my 2 cents, albeit general. The evap system is designed to operate as a sealed system with the gas cap on. There’s a canister, filled with charcoal, which absorbs/stores fuel fumes from the gas tank when the vehicle is not running. When you start your vehicle, a solenoid opens a valve and allows engine vacuum to suck the fumes from the canister into the intake to be burned. I can’t see why this would have any effect on engine RPM/timing unless there’s a vacuum leak somewhere evap system. Timing varies to advance or retard spark advance based on RPM (the higher the RPM the more advance) or if spark knock is detected (via the knock sensor) timing is retarded to eliminate/reduce detonation. Typically, engine timing is somewhere between 12 and 34 degrees depending on RPM. Older vehicles used the distributor’s static advance setting, engine vacuum and spinning weights to set and control timing. I believe your engine still uses a distributor, but I don’t think it does much (if anything) to influence timing as timing is adjusted by the PCM based on data from the crank, cam position and knock sensors as well as RPM.

I think I would disconnect/plug the evap system at the intake and see if that brings down the RPM and timing. If it does, then that proves that there’s a small vacuum leak somewhere in the evap system (a much smaller leak than when you remove the fuel cap). A large leak should throw a code altough perhaps not immediately.

Good luck.

 

[mbruffey]

I think I would disconnect/plug the evap system at the intake and see if that brings down the RPM and timing.

Thanks koda. I'll report back after trying your suggestion. I have to catch it at the right time, as it's intermittent. mbruffey

 

[mbruffey]

koda, I choked off the large vacuum line going into the intake as you suggested, without any noticeable effect. I think that, since the tank is building vacuum and no codes are thrown--not even small leak code--the problem is most likely not in the evap system. I'm thinking perhaps fuel pump, but first I'd like confirmation that the 100% fuel pump duty cycle at idle is odd. I have not been able to verify that suspicion. One other possibility is the crank position sensor. We have lots of salt up here in MN, of course; maybe it's gone flakey. But I'd expect a varying irregularity with the timing, not a fixed 25 degrees. mbruffey

 

[koda2000]

well at least you've eliminated the evap system from the puzzle. crankshaft position sender is an interesting thought. i guess the camshaft position sender could also be suspect, but i don't have any input on how these might be advancing timing unless they're giving the wrong signal to the pcm... good luck and let me know if you solve it.