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GMC Truck Runs Ratty

Q. Dear Vincent, I own a 1988 GMC 1500 with a 4.3 liter, 5-speed manual transmission. It has a rebuilt engine with 30,000 miles (206,000 total), and a rebuilt transmission. It always starts without hesitation, and in general runs very well.

My problem is after the truck sits overnight, or during the day while I'm at work, it hesitates severely on acceleration, a complete lack of power when you push the gas. The service engine light then appears, throwing a code 44. This happens until the engine warms up to between 120-150 degrees (190 degree thermostat).

If I allow the truck to warm up for 5 minutes, it doesn't do it. Cold, rainy or damp weather seems to worsen the problem.

I've taken the truck to numerous shops without any satisfaction. They all see the problem, but haven't any solid answers. So far I've replaced the following:

  • Coolant Temp Sensor - OEM
  • Plugs - OEM
  • Rebuilt and cleaned the throttle body
  • Checked for vacuum leaks - has 10"
  • Changed PVC
  • Changed TPS - OEM
  • Changed O2 sensor - 3 times - shops recommendation
  • I swapped computers with a 1990 GMC 1500 - used my Prom and Calpak
  • I haven't changed the EGR valve, it was replaced three years ago when the engine was rebuilt, NOT OEM

I have been reading a lot of your Q/A's and came across one where you mentioned a GM issued TSB in May 1988 on a revised PROM for certain driveability problems. I called the dealer and the GMC 800 number, they both said that I needed a TSB number. I was wondering if you knew the number, or may know where I might find it. This truck really has me stumped, I'd appreciate any advice you may have.

Thanks,
Cecil

A. This is the TSB you want. I hope it helps.

GMC NUMBER: 88-T-119
GROUP: 6E - Engine Emissions
DATE: May, 1988
CORPORATE NUMBER: 836517

SUBJECT: Driveability, Exhaust Odor, Inspection/Maintenance Testing

MODELS: 1987 And 1988 C/K, R/V, G, And M Trucks Equipped With 4.3l Engine (Engine VIN Code Z)

This bulletin cancels and supersedes GMC truck bulletin #87-t-107, dated December, 1987, all copies of which should be destroyed.

This bulletin expands the application to include 1987 and 1988 C/K, R/V, G, and M trucks, all 1987 M vans, and 1988 M vans prior to GMC VIN 523374.

NOTICE: 1988 M vans built after these VIN's are equipped with an oxygen sensor located in the exhaust y-pipe and an appropriate CALPROM. Calibration proms are particular to oxygen sensor location and should not be interchanged. M-van calibration proms listed in this bulletin are appropriate for vehicles with the oxygen sensor located in the exhaust manifold only. Also, a bleed connector should not be installed in a vehicle equipped with an oxygen sensor located in the Y-pipe as an over rich condition with increased sulfur odor and degraded driveability may result.

Some 1987 and 1988 trucks equipped with 4.3L engines (VIN Code Z), including 1988 M vans with VIN's prior to those listed above, may exhibit one or more of the conditions listed below.

Driveability Conditions:

  • Driveability sag or hesitation during the first five minutes of operation after a cold start or within one minute of operation after a hot restart.
  • Surge at speeds of 40-50 mph during a cruise or light throttle application.
  • Long crank time or false start after a 1-3 hour hot soak.

Inspection/Maintenance Testing:

  • Difficulty in meeting state exhaust emission inspection/maintenance testing requirements.

Exhaust Odor Condition:

  • Hydrogen sulfide smell typically described as "rotten egg smell" at idle.

I would check the base ignition timing, 10" of vacuum is kind of low for that engine. I would expect to see more like 15 to 17".

Additional Information provided courtesy of ALLDATA

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