1995 Buick Lesabre Custom surging

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I have a 1995 Buick Lesabre Custom with approximately 296,000 miles (I do a lot of hwy driving) on it. It has an intermittent (although now it seems to be heat related) problem of "surging" under load (ie: going up a hill or accelerating from stand still or cruising on hwy). These symptoms occur after the car has been running for 30 to 45 mins and the engine compartment gets fairly hot. After it first happened my mechanic did a complete tune-up - spark plugs/wires/air filter/pcv valve/fuel filter. This seemed to solve the problem for about 2 days when the exact same symptoms returned. This time he said to change the "PCM" - I did the changeout of this including moving the prom from the original computer to the replacement - after this the car ran well for about 1 week (I typically will drive almost 200 miles a day). I then was told that the ignition coils need to be changed - completed - no difference. About 8 months previous to this I had the fuel pump and filter changed in the gas tank as well as the EGR valve. The symptoms that required this change were different than what is happening now (just a side note). I would really appreciate any input on this as it seems to me that there should be a more logical approach to troubleshooting this problem than lets change this part and see if it works... I'm into this for over 400 dollars with no real progress towards a solution. Hope there is enough info on what was done and the result - please let me know if I can clarify anything.

PS: I'd also like to say THANK YOU ! to all the VETS in this forum. I appreciate the sacrifice made to defend our country and it's freedoms!

posted by  billr

Billr, I'm not going to flame you for the lack of introducing yourself in the apropriate section because you have presented your problem in a coherent manner. Still an intro would be nice.

My very first question to you is, has the PCM been scanned for codes and if so what are they. From your description it sounds like your "mechanic" is a is a parts changer and doesn't have a logical approach. At any rate let me know about the codes and we'll take it from there.

posted by  vwhobo

Thanks very much for your quick response !! Sorry about the info - will fill it in later I'm not involved in forums very often so would appreciate some patience if I lack in the proper etiquete - Thanks - Back to the codes - I have not had it scanned since we originally replaced the computer so, sorry don't have the codes (if any) but, will get them from my mechanic. I suspect I won't be able to get this info until monday sometime since he doesn't work on Saturday or Sunday and I can't get to the shop till next week - will post what info I can get then. Another side note that kind of puzzles me is :ohcrap: why does it go away for a few days after every "repair" ??

posted by  billr

just my :2cents: but i think it could possibly be because when a new piece of electrical software/hardware is installed it takes a while for the ECU to "learn" about it, and in the meantime it runs on a back-up system (open loop) which it reads from it's ROM maps which would explain why it was running fine. then as soon as the computer has become familiar with the new software or hardware it starts to run on a closed loop system which means that is moniters the engine and determinds what to do accordingly, in this way it may be reading the info wrongly or may not be recieveing the right info from its sensors and would start to under-perform, but to be able to say definatly the fault codes are needed...i hope i helped in some way or another... :wink2: :thumbs:

posted by  R34RB30DETTV

Thanks very much ! A friend had mentioned that something like that had happened with his minivan computer but, I wasn't quite sure if it was a stretch or not but the way you explained it makes complete sense to me.
Will get the codes as soon as I can and Thanks again to all !!

posted by  billr

really your mechanic should have run the system in and then done an assesment to see if it was operating correctly, but like VW said

..... later :thumbs:

posted by  R34RB30DETTV

Hi, Thanks to all - I have finally gotten a computer and have scanned for codes - however there are no codes listed in either history or current. I have since been able to get the car to lope and finally stall after running for at least 30 minutes in the garage (I had previously only been able to duplicate the problem while on the road). I have the computer hooked up but still get no error codes from the sensors. While using the datastream mode I can see the Heated O2 sensor varying from as low as .07 to approximately 90 millivolts. The car begins to stall and lope when this sensor is down near zero for any length of time (ie: 10 or 15 seconds). The problem is that when this sensor is removed from the circuit the ECM goes into open loop mode so I can't confirm that it is the sensor or something else causing the car to stall. My suspicion is that this sensor is causing all the problems since in open loop mode the car performs better than it has for quite a while ! I have been using a Haynes manual which shows a different HO2 sensor than what my car has installed - mine has 4 wires while the one they show has only 1 wire. The computer also indicates HO2 crosscounts - are these sensed through the other wires on the ho2 sensor ? Thanks for any and all of your help !!

posted by  billr

There is no such thing as a heated O2 sensor with only one wire so simply disregard the picture. If the datastream shows it sitting at .07 vdc for any length of time that tells me one of two things. A, you have an engine management, fuel delivery problem or vacuum leak that is driving it lean and causing it to stall or B, you have a O2 bad sensor that is reading lean to the PCM thereby driving the mixture rich and causing it to stall. Without further review of the datastream thats about as far as I can go with that.

Because you say the O2 sensor is reading .07-.90 vdc let's assume the sensor is good. Normally a bad sensor with hang in the .45-.55 vdc area. I'd say it's time to check fuel pressure when it's in the process of dying. There are other possibilities but it's still tough to do online.

posted by  vwhobo

Thanks again for all of your help and I apologize for the extended delay in response to your assistance. I finally was able to get a fuel pressure guage and watch the pressure - You were right on the money with the pressure dropping out.

1. My Haynes book says pressure should be between 40 and 47 psi at the schrader valve on the fuel rail. I read 37 solid - however when I remove the vacumn line from the regulator the pressure jumps to 44 psi. Vacumn guage indicates 18 hg with no fluctuations.
2. After the engine runs for at least 35 to 40 minutes to get the engine compartment temp up the pressure becomes unstable and drops to 20 and sinks occasionally to zero when the engine finally stalls. At this point removing the vacumn line has no affect. Tapping on the regulator with the
handle of a screwdriver has no affect on the performance of the regulator.

At this point I still am unable to find the relay or fuse for the fuel pump in the gas tank. The "relay center" in the engine compartment has no markings or indications for which relay is for which system. I'm sure there is a way to remove these relays for checking however the method still eludes me. :banghead: Any help with indentifying/removing these relays would be greatly appreciated!!! It appears that the problem lies with either the pressure regulator or the fuel pump relay failing under temperature stresses in the engine compartment. Not sure if I mentioned this but the car has no problems running when the air temp is down below 40 deg F.

Thanks again for you input and assistance in tracking this down - You've both been a BIG help!! Brgds :thumbs:

posted by  billr

I have a buick lesabre custom 91 with the same problem, all was caused by the computer that didnĀ“t work properly and wiring that presented fault contacts in some sensors or cause low voltage readings, but all began when I forget to connect the main fold air temperature sensor, this caused that air recycler vavle receive a fault reading and therefore over heat the engine and with the consecuent caos, all as you descrive

P.D. Apologize by my english that is not very good

posted by  rolp

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