Stalls when stopping and fluctuating idle - GM Forum - Buick, Cadillac, Chev, Olds, GMC & Pontiac chat


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Old 11-16-2010, 02:30 PM   #1
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Unhappy Stalls when stopping and fluctuating idle

1994 Bonneville SE, did all the hunting I could in the forums, and let me start by stating what I've done recently.

Early this summer, the gas tank developed rust-through (on top) and I ignored it until the fuel pump went, later in the summer. At that time, the weather was still warm, and at times the engine would occasionally stall out for no reason. I didn't get a chance to check if the problem was fuel pressure before the pump croaked.

So, fuel pump, sender, tank, straps, filter, and some of the fuel lines got replace by yours truly.

Incidentally, the straps and tank were Spectra Premium from Advance (ordered online, shipped to the house), fuel pump Carter, and sender Spectra Premium from RockAuto. The first tank came without a lock ring and my local store made good on that. The tank, after installation, leaked fuel from a small crack in the bottom of the tank, so it went back to my local store for replacement. The straps were both the same length, while OEM straps were unequal lengths. Both replacement straps were the same length as the 'short' OEM strap, so I had to use an extra-long bolt on the driver'* side to get the length.

I have still not tested fuel pressure.
During the process of dropping the tank, I lowered the back-end of the exhaust and the pipe broke at the cat. It'* still attached, but only by a short piece at the bottom of the pipe.

I have replaced the TPS (Throttle Position Sensor) - from Advance, and the UIM gaskets which helped with drivability (car was sluggish), and fixed the coolant leak under the throttle body, but idle still fluctuates, especially when first coming to a stop. After this happens a few times, the PCM seems to shut off the alternator, the voltage drops slowly, and eventually I can't restart the car.

Codes are 101 (MAF - I think I induced that by pulling the wire while the engine was running), 131, 134 and 171. The 171 is the most persistent, the others just recently started.

Hood was open for weeks while I was replacing the tank, with lots of rain on the exposed engine parts.

The terminals on the MAF are somewhat corroded, and I tried cleaning them up with a wire brush last night to see if that helped. It seemed to, but then the engine died and I couldn't restart (batt drained again), and this morning the fluctuation started again.

I've swapped the IAC with one off another used motor, no significant change.

I have a scantool that works with the obd 1.5 setup, and the FSM.
I guess right now, this has been irritating me so long, I just need some encouragement!

Should I try the MAF off the spare engine too/first or try welding up the exhaust pipe at the cat first? Or, other suggestions I'm quite open to all.

I didn't have the fluctuating idle/stalling/dying battery issue before the tank/pump croaked.
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Old 11-16-2010, 02:43 PM   #2
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Paul...I'm not positive in your year, however I'd start by trying to unplug the maf. This should make the car run off preprogrammed tables if it works like the 96+ pcm that I'm more accustomed to working with. If not, I'd hop right on cleaning those connections and or getting a newer plug/connector for the harness side.

The exhaust having little back pressure can also be messing with the system. Sometimes if the downpipe is too short this can cause the O2 to read improperly and that will send fuel trims into a tizzy.

Lastly.. if you can, please test fuel pressure. I'm confident that the tank, filter and pump should be up to speed. The question of the fuel pressure regulator is out there though. Being that it was used to the weaker old pump and seeing the new pump suddenly. It might have bitten the farm.
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Old 11-16-2010, 06:18 PM   #3
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Default MAF testing

Bill,

Tried unplugging the MAF when the engine was running, as per FSM suggestion, but engine died immed. Tried restarting and it ran, but no better. That'* I'm sure when the 101 code got set.

I'll try that again tonight, plus the fuel pressure tests.

If all that is good and I'm still faulting, I will try some hi-temp tape on the exhaust joint and see if that has any effect. Actual welding is still a few days off...

Incidentally, does PCM control the Alternator field? For grins I unplugged the connection (left Batt wire on!) and no output. My alt wiring diagram isn't specific regarding the origin of the signal on this lead, though it can be shorted to positive (with a fuse) for testing purposes. I'm planning on digging out an old ammeter to more closely monitor it, and fabbing a jumper when it apparently goes down to see if it is LOS from the PCM or an internal fault.
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Old 11-16-2010, 10:18 PM   #4
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Ok, so tried unplugging MAF with key off, started up, warmed up. Ran kinda funky at first, then smoothed out. I watched the LT and ST as the engine went into closed loop, and all looked good. Then, funkiness happened again. Seems like all of a sudden, no warning, loop status goes to open, rpm'* drop and LT goes up to 159 all at the same time.

Checked fuel pump pressure 'static' and seemed OK, though the pump ran and ran and ran, about 45 lbs PSI. Turned off ign, pressure at about 41-2 PSI. Tried to start car, but batt to flat to start after all the fluctuating idle. I swear WalMart doesn't know how to test a battery! Momentary CCA is one thing, but how many amp-hours will the darned thing hold?

Can't charge it now 'cuz it'* raining.

What'* next, duct tape the exh. pipe? See if the readings hold OK until the thing burns/blows out?
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Old 11-17-2010, 02:17 PM   #5
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Default Evap canister

I tried unplugging the evap solenoid before coming in to work this morning. No difference in operation was noted.

I have noted the EGR position always seems to read 16%. Mebbe another thing to check? I'm still working down the FSM checklist, but haven't tried taping the exhaust yet.
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Old 11-28-2010, 07:43 AM   #6
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Update: Car is still running poorly.
Walmart warranteed the battery (finally) and so now at least I can restart it when it dies. Alternator is fine, probably the original was too.
I noticed that it seems to idle best in 'closed loop', but as soon as it reverts to 'open loop' the fuel trims run back up and the idle starts to rise and fall again.

Any other ideas guys? Help!
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Old 11-28-2010, 10:15 AM   #7
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Are you getting o2 sensor readings? Do you have an exhaust leak before the o2 sensors?
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Old 11-28-2010, 08:33 PM   #8
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The O2 sensor does show various readings, and there is no exhaust leak before the sensor, as it is in the manifold. I had replaced the backside manifold last year, because it was cracked. The exhaust pipe is cracked behind the resonator, which is right behind the cat on my car. So, there'* little chance that fresh air is making its way back up through the cat to the sensor.

The fuel trims seem to climb out of the 'normal' zone when the car is cruising in overdrive.

I'm wondering if the O2 could just be a bit 'iffy' or a tad 'off''. Other than a fuel injector balance test, which I don't have the right tools to do, I'm out of ideas.

I did eliminate the MAF sensor as a problem by driving with it disconnected, and the same behavior happened. The fuel trims rose when the car reached cruising speed in overdrive.
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Old 11-29-2010, 12:14 AM   #9
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Run another scan. Do a cold start. What is the Coolant Temp Sensor reading?? Drive it for a bit and see what it reads.

Also, are the fuel trims positive or negative??

Me wonders if the cat is clogged...
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Old 12-08-2010, 06:58 AM   #10
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*bump*
I'm afraid I haven't availed myself of the opportunity to do much yet, as my garage is torn down and the wind and cold has been quite wintry... for fall. The alternator I was using dumped; fortunately I picked up two more at the JY the weekend before that failure, so I'm still running.

I seriously doubt the cat is clogged, as there doesn't seem to be any backpressure. I've run engines with clogged exhaust systems, and I know what it'* like. The motor seems peppy enough at the upper power band, just the low end idle and such is poor. The stalling has not been much of a problem lately, but still I get the fluctuating idle which shuts the alternator down temporarily.

Before I give up, if the wind and cold here can give me a break before the end of December, I have to recheck all my battery cables, and since I've now failed inspection(!) probably replace the O2 sensor, which has never been replaced.
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