Heavy missfiring/surging/stalling. WTF! - GM Forum - Buick, Cadillac, Chev, Olds, GMC & Pontiac chat


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Old 02-28-2008, 09:35 PM   #1
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Default Heavy missfiring/surging/stalling. WTF!

Well, let me tell you what happened. Car is 1987 Buick Park Ave 3.8l

Yesterday, I decided to sea foam the car. Sucked in the entire bottle from the pvc valve to the throttle body. After that, i drove the car around hard to let the smoke clear. This is when the problem started. It would stall/surge from 2500-5000rpm and would completely stall out if it idled sometimes. So I changed the spark plugs, gapped 0.045, cleaned the contacts, and thought I was good. Wrong.

Now I have a car that heavily misfires (i can feel/hear it), and like surges...its weird. As a bonus, it will also stall out, sometimes when driving, and usually when i come to a stop. When at a stop, it will just die...almost like turning off the ignition

I have no idea what the problem could be. I thought it might be the coils, but I replaced it with a used one i had lying around (im pretty sure its working) and the problem is still there. Weird thing though is the check engine light has yet to come on.

Any ideas on what this could be?
Thanks
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Old 02-29-2008, 12:12 AM   #2
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A Whole Bottle of SeaFoam? The can says to use only 1/3 to 1/2...

When you changed the coils, did you swap the ICM too?
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Old 03-01-2008, 09:36 AM   #3
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definetly sopunds like an ignition control module or a mass air flow sensor to me..

also check see if you find any corrosion under the battery boots. this sometimes is a big issue on alot of old cars that might give you the same symptoms that you are having..
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Old 03-01-2008, 11:45 AM   #4
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Default Re: Heavy missfiring/surging/stalling. WTF!

Quote:
Originally Posted by roman
Any ideas on what this could be?
Thanks
What you are describing sound a lot like a bad TPS sensor, but I wouldn't rule out the MAF. Point is, the car is having a hard time figuring out how much air is getting into the motor, which is where the hard idle is comming from...you can also make sure the PCV plumbing is back in place and everything is connected right. A car that surges is usually running very lean mixtures..the misfires can be caused by this as well.

Start by clearing your ECM (disconnect the batt ground for a few minutes) and check all the connectors your hands were around when doing the Seafoam thing. If the TPS got wet in the process, then check it'* operation. Now would be a good time to connect a scan tool up and look at the sensor values comming into the ECM.
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Old 03-01-2008, 02:44 PM   #5
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UPDATE

The following have been replaced

-spark plugs gapped to 0.045
-Plug wires
-ICM (One of them did not show the same resistance, so replaced with one with that showed the same resistance as all three)
-Oxygen Sensor
- I have a spare mass air flow sensor ill toss in just to be sure it isnt that, but the one that is in there right now is only 2 years old. I doubt its the mass air flow sensor

The problem is still persistent. The car runs much smoother when it isnt acting up. However, it still does the surging/knocking and the car still dies occasionally.

How do I check to see if my tps is acting up? Can I just spray it down with electronic cleaner stuff..like the same stuff used on the mass air flow sensor?
edit: How do I measure the TPS voltage? I have no idea where to rpobe the wires since the thing has to be connected to give me a reading

Could it possible be the crank position sensor? Or if that is out then the car wont run at all right?

I have a spare mass air flow sensor ill toss in just to be sure it isnt that, but the one that is in there right now is only 2 years old
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Old 03-03-2008, 10:04 PM   #6
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Quote:
Originally Posted by roman
How do I check to see if my tps is acting up? Can I just spray it down with electronic cleaner stuff..like the same stuff used on the mass air flow sensor?
edit: How do I measure the TPS voltage? I have no idea where to rpobe the wires since the thing has to be connected to give me a reading
There'* three wires going to the TPS. One is ground (Pin C), another is a +5VDC reference voltage (Pin A), and the last is the signal going back to the ECM (Pin B). It works just like a **** on a stereo.

You can test if it is OK by unplugging it and measuring the resistance at the connector between either Pins A & B, or pins B & C. When the thing is working right, moving the throttle by hand will cause the resistance to change. Resistance on pins A & C is a constant..it never changes with moving the throttle.

When one goes bad, you usually see places where the resistance goes to infinate as the throttle position hits a spot...this is the bad contacts inside the TPS hitting a dead spot, or the resistance changes in a non-linear manner. There'* no cleaning a TPS sensor out...pitch it in the trash and put a new one on if the sensor isn't reading right.

The units are supposed to be sealed, but as I've found, getting one wet is the kiss of death.

The impact a bad TPS has is pretty straight forward...the ECM has no way of knowing what your right-foot is doing with the gas pedal, or it sees a change in resistance that isn't related to the throttle actually moving to let more or less air into the motor. Drives the ECM nuts while it hunts for a pulsewidth that will keep the motor spinning.

It will not throw a code unless it fails completely (like unplugging it, or an internal short that runs the output signal past 4.9 VDC.)
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Old 03-03-2008, 10:35 PM   #7
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Went out to the garage and slapped a ohm meter on my TPS sensor to get some real values for you.

The resistance from Pin A to Pin C is a constant 4.7Kohm.

The resistance from pin B to C varies from 750ohm (throttle closed) to 4.5Kohm (throttle wide open).

Measuring the resistance between Pin A and B gives the same thing, only in reverse.

Moving the throttle by hand causes the resistance to change in a smooth, linear, manner, with no gaps. That'* what a working TPS sensor looks like.
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