SureTrack Community Fix: Slow To Get Going

Sometimes you don’t know if you’ve fixed a vehicle immediately afterward since it needs time to relearn. SureTrack community member MICHAEL2PINE17TRANSMISSIONS had a Chevrolet Silverado in their shop that had a problem accelerating. They posed the question to the community, and with the help of fellow community member Ericsautomotive and others, they were able to resolve the issue.

The following Real Fix summarizes the interaction between them in the community that led to the solution.

Vehicle
2000 Chevrolet Silverado 2500 6.0L

Title
Slow acceleration, replaced intake gaskets.

Complaint
Customer complains that truck is slow to accelerate but drives normally at speed.

Diagnostic Steps Performed/Parts Replaced
Test drive vehicle and it is slow to increase RPMs from a stop at any throttle position. No transmission codes. No engine codes. No misfires. The engine is capable of high RPMs except for launch. Fuel Pressure is at +/- 55 psi. We removed one of the precat 02s with no change. The air filter is clean and no blockages in the intake tube. The throttle plate is clean. No change when we unplugged in succession, the TPS, MAP, MAF. The precat O2s are not fixed lean or rich and switch with regular normal frequency with normal amplitude. The only 2 clues we can identify at this point are 1. the postcat O2s are fixed lean but do respond to added fuel, and 2. the engine load PID is consistently low as we have compared this value to another known good similar vehicle. I have read that a vacuum leak can drive the engine load input artificially low. With a vacuum leak throwing off the system, the ECU thinks there is less air entering the engine than their really is. This will mean that “load” values will be artificially low, which generally leads to timing being artificially high.

Best Answer from Community Member ericsautomotive
Smoke test the intake with the engine ice cold.

Cause
It passed the smoke test with no visible leaks. I had a diagnostic mechanic friend of mine go along on a test drive with a scanner. He also was inconclusive on what the cause could be. We opened up the exhaust between the pre O2s and the CATS. No improvement. We decided to pull the intake and inspect the gaskets. The silicone beads around the ports were absolutely flat. Two of the ports on the engine showed signs of inadequate sealing, no witness marks where the gaskets should have made contact. We replaced the gaskets and put it back together. That was late on a Friday before our vacation starter. On the test drive, I did not notice any improvement but I didn’t drive it long enough to let the trims relearn. The customer let us know yesterday that the vehicle was fixed. He had taken it to another mechanic to get a second opinion and that mechanic verified our repairs and agreed with our diagnostic steps. Strange how a smoke test did not reveal any leaks on those gaskets.

Correction
Replaced intake manifold gaskets. After long enough drive, fuel trims relearned and vehicle operates as it should.