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!
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.
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" ??
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:
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 !!
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:
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 !!
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
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.
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:
I have a buick lesabre custom 91 with the same problem, all was caused by
the computer that didn´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