93 TBI experts out there?

whitesnake01 · April 21, 2003

Hi --

I am struggling with a problem on my truck. The short version: It stalls on idle, most of the time.

The long version: 1993, 3/4 ton Suburban, 454, headers and a whipple blower. Have taken numerous strobes of ECM with my laptop. Problem started out as a oscilating surge on cold after the blower was installed. More recently, it has turned into a downright idle stall when the engine warms up. Basically, if I let go of the pedal it just dies. O2 looks fine. Plus are fine. TBI was recently rebuilt with no change for better or worse. Compression is 150-157PSI all around (about 1:10.5), with stock being 1:7.9. Barometer is fine.

One item that got my interest recently was the possibility that the IAC is out of whack. The shop manual talks about a require reset procedure if the TBI is ever disconnected. Not sure if this would apply, however.

Any ideas? Even crazy ones are welcome!!!

Blue Ghost · April 21, 2003

I would check:

A. Vacuum leak

B. base idle setting

C. IAC operation

Good Luck

americanmuscleman · April 21, 2003

Is it an idle only problem?

Does the engine run fine once your foot is on the gas?

How is it the rest of the time your driving it?

Does the idle seem low, or just like its missing?

Is it only when the engine is hot?

whitesnake01 · April 22, 2003

Thanks for your great help. Since I posted the note I have learned one additional piece of information that may be very relevant. In going over my captured numbers with Steven at Whipple, he was in particular surprised to see the MAP drop to about 16.6Kpa just before the engine died. In his view, that should never happen.

It would appear as I may be subject to an installation error of the blower. According to Steven, the MAP hose must be re-routed from the original fitting behind the TBI (which must be plugged) to the base plate below the blower. According to Steven there should be a fitting in the baseplate but on mine the holes were plugged with screws on both sides of blower.

Such a re-routing should certainly bring up the MAP values. Steven thought it was likely the ECM was trying to feed the engine for this weird MAP reading, when it was built to feed based on the MAP reading after the blower rather than before the blower.

While this is a problem with my setup, it is not 100% clear if this is what I am experiencing. I should have a fix installed in the next few days.

To answer some of your questions:

Q: Is it an idle only problem?

A: The particular symptom I was looking at now appears to only be related to idle .. or rather .. throttle at 0% open which sets the idle bit in the ECM's control register. I have had other problems with it too, including it running lean leadnig to detonations. The lean mix can also possibly be explained by the MAP reading.

Q: Does the engine run fine once your foot is on the gas?

A: Yes, I'd say so except for my previous problems with detonation. Some of the detonation problems (under very heavy load) can be explained by a partially clogged radiator that has since been replaced. But they may also have been releated to a too lean mix. I have not yet subjected the truck to very high loads and high temperatures since those fixes.

Q: How is it the rest of the time your driving it?

A: See previous.

Q: Does the idle seem low, or just like its missing?

A: Just very low to the point of stalling. When the engine is cold it has also experienced RPM hunting at idle, swinging from 450RPM to 1100RPM in a 8-10 second cycle until it warms up. Some of the low swings would occasionally kill the engine.

Q: Is it only when the engine is hot?

A: The stalll is primarily when it gets warm. I'd say that the stall at warm only started happening after I had the TBI unit rebuilt. I am not completely sure, however, if the TBI unit is to blame for it or if it is just excaserbating the problem.

whitesnake01 · April 22, 2003

Also, for anyone else out there with a 454 and a whipple blower. I have confirmed my cylinder readings with Steven. 150PSI +/- 7-8% is considered normal on this engine with the whipple blower installed.

As I pointed out earlier, the stock is 1:7.9 which should be about 117. This means the blower runs at about 5PSI boost during cranking for a compression test, if my calculations are correct.

whitesnake01 · April 30, 2003

Things are better but not perfect.

I have now relocated the MAP sensor input to below the blower instead of being on the TBI. After this, the engine ran very rich with no significant reduction in low RPM idles. I also saw the Long and Short term fuel trim levels being lower, meaning richer. Towed my 8,300lbs camping trailer 157 miles and saw occsaional plumes on black smoke when it shifted down under load.

Replaced PCV (as per suggestion in shop manual). No change. Replaced O2 sensor It appears much better. Has not stalled, but has gotten into low RPMs before engine warmed up while waiting at a stop light. What I do not like about the low RPM is the potential that the truck might stall as I make a slow speed turn when power steering drag is applied to an engine that barely is alive.

Will be towing the trailer back again this weekend, same distance. I hope that I can confirm the O2 sensor has fixed the generally rich behavior, even though the low idle may remain.

whitesnake01 · May 13, 2003

A little update. Have replaced the Idle Air Control solenoid. No change. After some discussions at work, someone suggested it may be something with the ignition timing. Replaced the Engine Coolatant Temperature sensor, which appears to have improved things. The theory being that too low resistance in this sensor will overstate temperature to ECM which in turn pulls back timing more than it needs to be.

But I did run into a DTC 43 condition, which basically claims the knock sensor is not working properly. This is a transient condition and is only reported for two reasons on my truck. Not working at all, which is not the case, as I do see a non-zero knock count since power-up. Or, failing a knock test around 80Kpa MAP (which the computer thinks is near WOT). This test is done by advancing timing and expecting a knock. If, after two attempts during near WOT it fails to make it knock, it raises DTC 43. With the blower on and the MAP below the blower, 80Kpa is not WOT, only about 55% of WOT.

Maybe I should just replace the knock sensor. Manual says it is on the passenger side of the block. But where? There is a sensor above the exhaust in the middle of the block. Is that the one?

Wingnut · May 13, 2003

Yeah, right by the starter.

Thats where the one on my 92 is anyway.

whitesnake01 · May 14, 2003

I just checked in my shop manuals last night and it is indeed near the starter for the 454. I picked up a new AC Delco Knock Sensor this morning for $43, or so. I'll swap that on Friday and should be able to tell if there are any differences after I tow the trailer this weekend.