Oscilating idle at startup of 454

[whitesnake01]

I am experiencing a strange problem with my 1993 Suburban with a 454. Before the engine warms up, the idle appears to be very variable.

 

I recently purchased a software package called Data Master, which basically is scanner software that runs on a PC. It can capture ECM counters at 10 strobes a second. You can then chart out the results and look at different mixes of counters.

 

I did a rather long test, which consisted of starting the engine cold at about 25F and then leaving it idle in the driveway until the engine reached operating temperature. For the first couple of minutes the engine RPM would go up to about 1450 and then drop down to 550 at an interval of 8 seconds or so.

 

When tried to cross-chart the numbers, it appeared as if the fuel injector pressure oscilated inversely in phase with the RPM. The injector voltage and pressure would climb while the RPM would gradually drop until the ECM appeared to pull back on the injectors, etc. Basically, it looked as if the RPM would work better when the pressure was lower.

 

This continued until the ECM started to adjust the short and long term fuel trim values, which happened after about 2 1/2 minutes from startup. Once the actual short and long trim values started to lean out the mix, the idle became steady. At that point, the engine's temp was about 89F, so the thermostat was not open.

 

When I ran the car, after that, it appeared as if the short and long term fuel trim values were on the lean side. The oxygen sensor has been recently replaced. It would appear as if it reports enough variety in values that I think it is working.

 

I am wondering if the problem is not that the TBI for this unit is malfunctioning such that it is giving the engine too much fuel. Consequently, a 128 trim is actually much lower in actual terms. So when it finally leans out the mix to 140 and above it may be reaching what should have been 128.

 

Any ideas?

 

Thanks,

[CMNTMXR81]

Did you look at simple things first?  Fuel filter?  Fuel pressure/pump?

[whitesnake01]

Did you look at simple things first?  Fuel filter?  Fuel pressure/pump?

The filter is new. I have not verified the fuel perssure. Do you think the pressure could be too high?

 

I should add these additional datapoints:

 

* These problems started appearing after I had a whipple blower installed on the truck. That kit came with a fuel pressure booster. It is more pronounced in cold weather. The truck runs fine, once warm, and does not appear to suffer from fuel starvation.

 

* I've seen occasional fuel leaks behind the TBI unit, not associated with the fuel or return lines. In the whipple setup for my configuration, the TBI unit is sitting at a 60 degree angle, with the "top" of teh TBI unit facing the driver rather than standing up. In this config, the the fuel fittings are facing down. I've seen drippings come out on the top of the TBI unit, the area facing down, by the air duct. This is the area of the TBI that would normally be flush against the air filter, not the area mounted on the manifold.

 

* I used to be able to smell gas from under the hood, after shutoff, before the whipple came on. But I never saw any drips.

 

With my recent problems, I started to surmise that the TBI may have gone gradually bad (perhaps some worn O-rings) and that the whipple setup just exacerbated the problems with it. Now that the TBI is leaning, what would normally "leak" back into the manifold could now be dripping out by the side.

 

What could a too high fuel pressure create in terms of symptoms?

 

Thanks,