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'90 Grand Voyager - stumbles when hot


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08-06-04, 10:49 PM   #1  
'90 Grand Voyager - stumbles when hot

I'm at my wits' end with this one! My 1990 Plymouth Grand Voyager (3.3 liter v-6, 4-speed auto trans, a/c, 140,000 miles) has developed an irritating habit of not responding to throttle input once the engine is warm. Seems only to happen under load; parked or in neutral, it responds just fine. The first time it happens, I can either be accelerating from a stop or cruising at a steady speed. The "stumble" goes like this: at any throttle position other than idle, the engine backfires through the exhaust (sometimes there's a muffled "pop" from the air cleaner box), and the engine won't respond to throttle again until it's about to the floor. Once the engine cools down, the problem disappears.

Within the past year, in an attempt to remedy this problem, I've replaced the water pump, thermostat, radiator, coolant temp sensor (to PCM), TPS (twice), MAP sensor and fuel pump. All I seem to have accomplished is to alleviate an overheating problem, which was bad enough to warrant repairing on its own but which I also thought might be contributing to the hot stumble. I'm not above replacing the fuel filter and/or fuel pressure regulator, but I can't see how those components are going to affect driveability when hot.

Any input on this problem would be greatly appreciated, as I'm running short of patience, will and funds. By the way, a similar problem on my '94 Ford pickup was remedied by replacing the inoperative fuel pump in the front tank and running only on that tank. However, as I mentioned, the current pump is less than a year old. Thanks in advance - Chris

 
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08-07-04, 07:02 AM   #2  
McDann
Did you forget to mention any repairs or maintenance to the ignition system or have you overlooked this as a possibility?
I see the problem you described quite often being caused by excessive resistance in the ignition secondary.

 
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08-07-04, 08:10 AM   #3  
Thanks, McDann, for your reply. No, I have not even looked at the ignition system as a possible cause of this problem, nor have I checked or replaced any parts other than routine spark plug changes. I suppose the wires and/or coil pack could have broken down over the years and miles, and could even cause a problem like this that only shows up when the engine is hot. But I wouldn't have thought that such a problem would be choosy about throttle position. I'll look into it, though. It may be in a Chilton's manual of mine, but if not, do you know what kind of resistance I should see through the wires and coil windings?
I thought I had replaced the exhaust gas O2 sensor since I've had the vehicle, but in retrospect I think I was mistaken. What's the chance THAT'S what's giving me fits? It's the heated sensor, and I don't know for sure whether those have a regular replacement interval. I've heard that the non-heated variety should be changed at every tune-up, or at least once a year, but nothing on the heated ones.
Again, thanks for the input. I'll post with resistance results soon. - Chris

 
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08-08-04, 08:56 AM   #4  
Okay. I checked the resistance through the spark plug wires, and here's what I found: 4K ohms on the shortest one (about a foot), and 10K ohms on the longest (about two and a half feet). Resistance through each pair of coil towers was 12K ohms. Doesn't seem too bad, but I'm not sure what the actual specs are.
I posed the problem to one of my co-workers, and he said he had the same problem on a similar vehicle, and that it didn't go away until he replaced the idle air control motor. He said the assembly was so carbon-fouled that the plunger would not move in or out, but that the new part cleared it right up. Does this sound plausible?

 
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08-08-04, 06:50 PM   #5  
ezgrinz
Maybe what my Voyager has taught me will help you: The O2 sensor actually puts out 0.5-1.0 volts in operation (rich-lean); By about 60K miles, it will pay to replace if it has yet to fail completely. Have you checked the computer error codes yet? This can help identify a bad sensor or where to look first ... but on this old series, many times the error codes appear only if the sensor is dead, not that it's "just" malfunctioning.

Regardless, I can tell you that my problem with coughing spasms during accellerations or cruise spees ended with EGR (includes transducer). There were also significant improvements after the AIM valve/motor and the TPS; but EGR alone eliminated 90+% for me.

Other specific possibilities include the fuel pressure regulator, a partially blocked fuel filter, a constricted fuel line, and a failing distance sensor. [Mine's running great now -- except for suddenly dying during idle. I guess it's time to suck it in & have my Voyager's sustained fuel pressure checked. Good Luck with yours.]

 
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08-08-04, 07:34 PM   #6  
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I would say plug wires are your problem. Testing the resistance with a meter will not be very accurate as the resistance will change with a temperature increase or decrease. I may be wrong on this but if I remember correctly this was the case when conducting electrical current.

 
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08-09-04, 11:37 PM   #7  
Thanks again, guys, for the input. Can't do anything about EGR, as this van was not so equipped. I won't say it's NOT time to replace the O2 sensor and plug wires, but as I was looking at the few remaining sensors I had not yet replaced, I found the intake air temperature sensor to be damaged. After replacing just that one sensor, I took it for a short drive, and the van behaved better than it had in a year. Well, I had also cleaned the idle air control motor, and had intended to check its function, but my work schedule demands that wait for another day. I'm going to drive the van for a few days, see how it does and report the results here. Until next time,... - Chris

 
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