TxBuicks

1991 Roadmaster Estate Wagon - Rough Idle

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I was driving my 1991 Roadmaster Estate Wagon for a while on the Interstate with no issues.  I pulled off at my exit and when I got to the light to turn left, it just died suddenly.  I was able to start it back up immediately, but the idle is so rough now. It will go down to where I think it is going to die, then rev back up to 2,500 RPM, then back down to where I think it will die again, then back up.  And it is 'chugging' all the while.  If I give it gas to go, it seems to run fine.  It has the throttle-bodied fuel injected 5.0 liter engine.  If it had a regular carburetor on it, I would think the float is stuck and it was flooding itself.  I don't know where to start looking.  Does anyone here have any suggestions?

 

Thanks.

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Sounds like a fuel pump issue. Not flooding but the opposite. Has the fuel filter been replaced recently? Has the fuel pump ever been replaced? Does the manual show where the gas tank is ventedn and can it be checked for obstructions? 

To the best of my knowledge there should aslo be a fuel pressure regulator. Not sure how to test that but its something to consider along with vacuum leaks.

Wish you good luck Roy. 

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No code thrown (CEL)?   I would venture a guess the throttle body idle control motor needs a cleaning.  Check the TPS. 

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Gotta remember that your Roadmaster is powered by a Chevy engine.  Google "GM Longroof forum" and see what those guys might have to offer. Buick Roadmaster wagons, Chevy Caprice Wagons, and Oldsmobile Custom Cruisers.  Lots of good information.

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It sounds familiar to an issue I had with a Mass Air Sensor in a 97 Chevy lumina with the 3100. I know different motors but that sensor can get dirty and affect idle as you describe.  

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The system he has does not use a mass air. It uses a Map [ vacuum] instead. AND that is one of the sensors indicated in the shop manual for the symptoms Roy is having.  Good call.

 

  Ben

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That's an OBD1 car and the first of the OBD2 cars will be 24 years old this year. Pretty close to the AACA definition of an antique car. Other than things you can visually check there are a lot if drivability diagnostics that can only be done with a scanner or specialized test equipment like a fuel pressure tester and the like.

Some things are routine like John's suggestion to change the fuel filter. The B-body GM cars have a bracket mounted on the frame rail that holds the filter at about a 45 degree angle, just right to capture water in the lower second. After 25 years you could be a couple of molecules away from a pinhole leak at any point in time.

The possibility of a failed or failing MAF, MAP, O2, or other 25 year old sensor could be pretty good. It is expensive to guess.

 

Those are good cars and reliable. At 245,000 miles you may want to get it on an upscale scanner and see what all the  controls are doing.

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Even Advance Auto and like have scanners to read codes on OBD1. It is just OBD 1.5 that is impossible without special scanners. (1994/5 some GM only).

 

The Check Engine Light did come on?🤔 If not, there is no code to decode....

 

Map is also my guess, unless a vacuum line broke/fell off.

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I replaced the Idle Air Control valve and it started right up and idled smooth and solid for the few seconds I had it on.  Convinced that this fixed the problem, I turned it off.  I drove it this morning to work.  During the first several stops, everything ran well and I was sure I had it resolved.  But about the 4th time I had to sit and idle, it started again.  Definitively not as bad as before, but still erratic, with the RPMs going up and down.  Rats.  I will check the MAP next. I have no 'Check Engine' light, but I will check for any codes tonight. 

 

Bruce, I have a shop manual specifically for a 1991 Roadmaster Estate Wagon, and spent an hour looking it over, but just got confused with all the emissions stuff.  1992 would be different because 1991 and 1992 had different engines.

 

Thanks for the suggestions, everyone.

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Roy...don't know if this will solve your RPM ups and downs, but I had the same problem in a previously owned Riviera.  By replacing an oxygen sensor the problem went away. 

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3 hours ago, TxBuicks said:

I replaced the Idle Air Control valve and it started right up and idled smooth and solid for the few seconds I had it on.  Convinced that this fixed the problem, I turned it off.  I drove it this morning to work.  During the first several stops, everything ran well and I was sure I had it resolved.  But about the 4th time I had to sit and idle, it started again.  Definitively not as bad as before, but still erratic, with the RPMs going up and down.  Rats.  I will check the MAP next. I have no 'Check Engine' light, but I will check for any codes tonight. 

 

Bruce, I have a shop manual specifically for a 1991 Roadmaster Estate Wagon, and spent an hour looking it over, but just got confused with all the emissions stuff.  1992 would be different because 1991 and 1992 had different engines.

 

Thanks for the suggestions, everyone.

 

Reset the IAC one more time.  See if she stays steady. 

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Sounds like your problem comes after the system goes to closed loop operation. That is where the O2 sensor can affect it. I think there are four of them, maybe just two, on your car.

The code readers will display a code if one is set and most will let you clear the code. To diagnose the specific part causing the problem a higher level diagnostic tool will plot a graph of individual sensors.

 

Diagnostic equipment for 1980' through 1995 is available at pretty low prices on Ebay. I bought an OTC 4000 for under $100 a few years ago. I just got a Foxwell NT680 for the newer cars. It was $500 with an accessory kit. For a hobbyist that's a low end tool, but those systems are here to stay and doing your own work gets expensive guessing.

 

There is also a learning curve. It's a lot like reading an oscilloscope with Windows on top.

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Well, it appears to have been the Idle Air Control valve after all.  I've been driving it for two days and the idle seems to get better each time. I guess the computer had time to make all of the adjustments.  Keeping my fingers crossed....

 

Thanks, again , for your advice.

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Check the torque on the TBI mounting nuts.  The OEM gaskets had a tendency to deteriorate, which caused a vacuum leak there.  New gasket fixes it.

 

Probably didn't hurt to change the IAC, considering the age of the vehicle.  It's learning to "cover" for any air leakage into the system, I suspect.

 

Certainly NOT a fuel pump or fuel supply issue!  A weak fuel pump manifests itself in "Extended Crank Time", as the engine cranks, the fuel pressure slowly builds until it hits the 54psi minimum and THEN it fires off and generally runs decent.  Can also be caused by a restricted fuel filter.  But it will NOT fire-off quickly in these failure modes.

 

NTX5467

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Comparing the long term fuel trim and short term fuel trim can be a good indicator for the AIC or a vacuum leak if you find someone with a tool to look at them. There was a point in time when having a timing light and a dwell meter were extravagant items in a home garage.

 

The last 30 years have seen a lot of technology go into cars. Makes me think back to the year I bought my father a pair of booted winter wiper blades for a stocking stuffer. I guess a code tool would be about as good today if he was still around.

 

Remember the guy who had a worn dime for setting points and a long screwdriver to use as a stethoscope to set timing?

 

Bernie

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Hadn't heard of the "worn dime", but remember reading about the "fold-over match book cover". Plus a normal paper clip to set the plug gap with.

 

NTX5467

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The old guys whom told me this stuff probably never did it, but had lied that they had so many times they believed they did... just like the guy who told them.

 

I hear there is a mandatory sticker required for Model T Fords "The Surgeon General warns this vehicle may be held together by bodily fluid hazards and potential puncture wounds."

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The worn dime is a real ignition tool. It had to be a silver dime,  preferably the Mercury style.

 

The clad dimes we have had since 1965 do not wear to the correct thickness to replace your point feeler gauge!😂  They are .053" thick. The silver ones will wear to about .020", good enough!

Edited by Frank DuVal (see edit history)
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