Intermittent Limp Mode With No Vacuum Leaks?

1995 CHEVROLET VAN

73,000 MILES • 5.7L • AUTOMATIC

ERIFE

MEMBER
2 POSTS

95 G30 5.7 w/160° therm 4L80 20 ft motor home.
Problem started soon after purchase, previous owner stated "nothing like that ever".
AC compressor clutch came apart 60mi on first voyage. Unplugged ac clutch then 20 mi or so went into limp mode, hi idle, trans limited to either 2nd or 3rd gear. Made it home replaced idle air along w/ac compressor, everything else checked out tpi, ecm temp sensor, etc. no codes no vac leaks, then O2 sensor didn't seem right so I replaced it(Bosch). Seemed to do the trick, drove locally several times, okay. 2nd voyage, apx 150mi limp mode. Various codes every 150-200 mi reset. Most common was analog digital error. While in Alabama talked to a guy that seemed to know and convinced me to change ecm temp sensor and ecm. On my return, same stuff every 200mi. I did get PROM error a few times along w/analog digital error. Another pro said O2 sensor needs GM part & junk the Bosch. Picked up a PROM but haven't tried it yet. Today O2 sensor circuit code and 85.

Jun 27, 2026 at 8:27 PM

STRAILER

✓Experienced Mechanic
55,051 POSTS

Code 85 is a PROM/MEM-CAL error on this era GM ECM/PCM. Analog/digital error also points toward an internal ECM problem, PROM/MEM-CAL problem, poor ECM power/ground, or voltage/reference issue.

The O2 code may be real, but it could also be a symptom if the ECM is getting bad information or losing stable ground. I would check the grounds at the thermostat housing for the PCM wiring harness first. Also, when you say limp mode when does the motor home do exactly and is it the same each time and what do you do to get it going again? Also, I have seen bad fuel pumps or contaminated fuel do the same thing FYI. It might be tough but can you please upload a short video in your response of the problem, so I can see what's going on. I would replace the oxygen sensor, also replace the thermostat with a 190 degree so the system will go into closed loop, this can be the entire problem. Let me know how it goes so I can get you back on the road.

Jun 28, 2026 at 10:16 AM

ERIFE

MEMBER
2 POSTS

Replaced O2 sensor, no real change.. found some real GM info, catalytic converter temp's we're off, removed. Still some trans issues. Exhaust temperature ran high, close inspection of EGR revealed vacuum leak internally in the EGR control module, replaced EGR control, fixed hi exhaust temperature but still had some hi idle and trans issues.
Here was the killer, TPS measurements -ohms were spot on 1-2-3-4-5 (relative) all the way to full throttle and back never missed a step, but! With key on voltage from the signal wire was apx 2.4vdc at idle, GM said it should be 0.2 idle. Replaced TPS everything is back to normal.
Theoretically speaking, TPS told ECM throttle was apx 4% = rich, idle air would open = hi idle , trans said we're not moving @4%mph so somebody's lying or we got communication issues we just going to save ourselves. Also created high exhaust temperature which failing EGR control could not control....,
Also discovered that OBD1 will tell you if a sensor shorts out or open circuits but it doesn't care about inappropriate readings within its range.
Thanks for GM's info.

Jul 30, 2026 at 7:54 AM

STRAILER

✓Experienced Mechanic
55,051 POSTS

Glad you could get it fixed, thanks for letting us know. Please use 2CarPros anytime we are here to help.

Jul 31, 2026 at 8:28 AM

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