Mechanics

CHEVY LIMP HOME MODE NIGHTMARE.

1999 Chevrolet Suburban

Engine Performance problem
1999 Chevy Suburban V8 Four Wheel Drive Automatic 76,461 miles K1500 5.7 L Vortec Engine

Engine will start and runs fine when cold.
After1-2 miles driving engine, it will not come up to
speed. It backfires through the intake as if lean fuel air mixture, bogs down and no power. I've read about a "limp home mode" that seems to fit what the car goes into. I shut the engine off and it immediately restarts and everything fine for a few minutes and then it reverts back to same problem.
Fuel pump first replaced Dec 5-08 after showing signs of being noisy and engine not starting one day. This new pump became noisy after a couple of days and then lost power on December 25th. It was then replaced Dec 29th with a fuel pump with a different style with modified connector. There have been problems with the original fuel pumps. The fuel filter changed again, 60-65 psi fuel pressure at test port. Fuel pressure regulator, spark plugs, cap, rotor and cool sensor replaced. Intake manifold gaskets replaced. O2 sensors replaced at shop after their codes read needed replacing. The shop not able to diagnose, only that the intake manifold had a severe leak that was then leak fixed at our home. The shop scan codes read P0131 O2 low voltage P0151 O2 low voltage, P0405 EGR Sensor "A" Circuit low, P0102 Mass Air flow low input. The Mass Air Flow sensor was cleaned with "Sensor Kleen" before going into the shop. This nightmare has been going on for almost 2 months. The next step might be to replace the Powertrain Control Module (PCM), I have been reading that all too often technicians tend to blame that which they understand least and if the engine is not running right and the cause is not obvious they may blame the computer. No codes are showing from the hand held scanner we put on the car after taking it out for a test drive yesterday. My husband is retired and our funds are getting low for fixing this vehicle. We would appreciate any help you could give us.


http://www.2carpros.com/forum/automotive_pictures/310266_A_picture_of_1999_Suburban_Feb09_1.jpg

Avatar
Burnie
January 30, 2009.



One other possibility is that the CAT (convertor) is clogged or partially clogged.

If CAT is bolted (vs welded) remove pipe to CAT and inspect the CAT. Look for melted material, cracks inside the CAT.
If CAT is welded, have a backpressure test performed to see if CAT is clogged.

A clogged CAT will cause the symptoms you describe.

Mmprince4000
Feb 4, 2009.
Thank you for your response.
CAT (convertor) checked and not the problem.
During the test drives, everytime
the car will start right back up after
the ignition is turned off. It has lots of acceleration and power for another 2-3 min. And the same cycle again after turning the ignition off. Could you describe the " Limp home mode"? We are not sure if that is what is happening with the car, only that it sounds like what we have read about.
How can we tell if the Powertrain Control
Module is to blame? The scan tool shows no problem codes after test driving or anytime. Also, it shows no problem codes even when two or three engine sensors are disconnected. It will generate a code when the fuel cap is off while testing. Check engine light has never come on. I look forward to any light you can shed on this problem. Thank you.

Tiny
Burnie
Feb 4, 2009.
If you disconnect a sensor and the PCM does not recognize that it has been disconnected, then the PCM may very well be the problem.

Limp mode will revert to a " default" set of paramaters (fuel, ignition), ignoring real time data. This is generally a problem with the PCM or a communication error. This " default" is always rich fuel and retarded ignition timing to prevent engine damage, It is intended to get you to a repair facility at reduced power and performance.

There are 4 types of error codes, " P" (powertrain), " C" (chassis), " B" (body) and " U" (CAN or contoller area network). Most DIY scanners will only scan " P" codes.

So you may have to go to a dealer or trusted garage and get all the codes read.

Mmprince4000
Feb 4, 2009.
Thanks for your reply. We had it into the shop on
Jan. 16th and they ran a scan. It showed
PO131 O2 Low Voltage, PO151 O2 Low Voltage,
PO405 EGR Sensor " A" Circuit Low, PO101 Mass Air Flow low Input. They replaced both O2 sensors and said they
couldn't diagnose any further until the intake manifold leak was fixed. My husband has fixed the leak and replaced the gasket but we are still having the same problems. So far over $900 in parts and $467 to the shop and we still have the same problem. I had it out for a test drive yesterday and was able todrive 3 miles at 40 miles/hr before it started to lose power. I parked it for 20 min. And then started it back up and lost power after driving a short distance. After it stalled I put it in neutral and started it back up, without turning off the ignition. This time it would only go a short distance before losing power. The next time I turned off the ignition and got further but only a mile at a time before having to shut it off. It seems that when the ignition is turned off, the computer reboots. Does this make sense? Would that indicate the PCM would need replacing? Thanks again for your help.

Tiny
Burnie
Feb 5, 2009.
If intake leak has been fixed, take car to any autoparts store for a free scan.

Pay attention to the EGR and MAF sensor codes, either which could cause the problems you describe.

Mass Air Flow, measures volume of air entering engine and sets fuel and ignition maps accordingly.

EGR (Exhaust Gas Recirculation) meters burnt exhaust gases into combustion chamber.

Mmprince4000
Feb 6, 2009.
Thanks for the advice. We used the scan
tool and it read two pending codes,
PO131 and PO151. These are the same codes
that the shop read and then they replaced the two oxygen sensors. We have two choices if we replace
the PCM. Either buy a rebuilt one and have it reprogrammed ($400+) or send ours back east and
have it reprogrammed ($250+). We live in Canada
so I don't know if we could buy the PCM in the U.S.
We still don't know if replacing the PCM will cure the problem. Thanks again for any help you can give us.

Tiny
Burnie
Feb 9, 2009.
We have solved the problem after several months.
It turns out that the 2 Napa fuel pumps that we installed were the source of all the trouble!
My husband put the old fuel pump back on and
the car was almost back to normal.
We bought a new GM fuel pump and then installed that and the car has been working fine ever since.
Hope this will help someone else down the road
with the same problem.

Tiny
Burnie
Nov 12, 2009.
Glad to hear it's running again, and thanks for using 2carpros.

Mmprince4000
Nov 12, 2009.