High idle help after replacing fuel pumps, 1988 K30, Original, TBI, 454.

So I lifted the bed and replaced both fuel pumps last week, after putting it all back together I now have a high idle issue and not sure why (vacuume leak?) Removing the fuel lines from the sending units is a bear with the tank In place. I wonder if I put a hole or tear in one of the hoses, there are 3 to each sending unit, I noticed on my older trucks the middle tube on the sending unit is capped off. TBI trucks use that tube. I did no work under the hood, everything was underneath the bed.

suggestions?

I'm going with it's a coincidence. A leak in the lines you messed with will only cause a fuel leak, or just a vapor leak in the vent line.

How long did you let it run? A tbi motor will often have a high idle on a cold start for whatever reason. Especially the 454 models. It's often a bit excessive too. Otherwise, it could just be a simple vacuum hose under the hood that has rotted or maybe a critter chewed on it.

Thanks, I agree, this truck has always had what I consider a higher than normal idle. I used to disconnect the idle control solenoid and it worked great.

thanks for the input

Still a high idle, I’m thinking about doing this? Anyone else done it on a TBI set up?


https://itstillruns.com/reset-baseline-idle-gm-tbi-7959677.html

roguewave said:

Still a high idle, I’m thinking about doing this? Anyone else done it on a TBI set up?


https://itstillruns.com/reset-baseline-idle-gm-tbi-7959677.html

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That works, as long as the IAC valve is properly functioning. The IAC valve could actually be your problem too.

I guess what I should have said was, test the IAC valve and circuit before setting the base idle. Because if you try setting the base idle when the IAC is on the fritz, it's never going to properly compensate the idle and you will likely have intermittent idle speed issues.

So if you have checked the usual stuff like vacuum leaks and sticking throttle, here's a flow chart to test the IAC and circuit. Please note that this diagram is specifically for a 2.5 tbi motor in a car so the pinouts may be different in the harness connector, but the operation and test is the same. (I simply had this service manual handy)

bucket said:

That works, as long as the IAC valve is properly functioning. The IAC valve could actually be your problem too.

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replaced IAC last year I think. Is there 2 serviceable parts on the right side of the TBI/carb? the IAC and something else?

Another thing to consider is the tbi fuel pressure regulator might be starting to fail. New pumps may have been enough to cause symptoms. it’s a long shot and less likely than other things but if the tbi is original and the regulator has never been replaced it should be checked if other trouble shooting does not solve your issue.

roguewave said:

replaced IAC last year I think. Is there 2 serviceable parts on the right side of the TBI/carb? the IAC and something else?

Click to expand...

If it's been replaced in the past, it could still be a problem. They don't always last very long.

Oh yeah, check the throttle shaft too for slop. That can cause intermittent variances in the base idle setting, which makes it hard for the IAC to properly do it's job.

The throttle position sensor (TPS) can cause a high idle, but that almost always is associated with other driveability issues as well. A quick test is to simply unplug the TPS while the engine is idling to see if it slows down.

I have the same set up in my truck only its a 91 year model. I too, was concerned about the high idle speed upon starting the motor. I did some reading about the high idle and all the different sensors that the engine has and is sending to the ECM. Like you, my first thought was the IAC. But after reading about the whole system of how the ECM controls the motor, it wasn't the IAC.
When that unit is not working properly the engine surges during the idle range. What I found is, that's the speed the ECM wants to run the engine on a cold start. I've also have determined that 454's are cold blooded, then when warmed up they get very warm. Mine, if left idling, will take about fifteen to twenty minutes and it will kick down, on its own. Mean while, no amount of kicking the gas peddle will get it to throttle down.

I did discover another issue with the motor and that's the fan clutch. Its been talked about not so long ago in one of the topic heads but right now I couldn't steer you there. So what was going on with mine, its was frozen. The fan blades would not turn on the fan hub freely, with engine off of course. So when I started the motor the fan blades were under full power of the motor. I have the stock fan blades by the way. Because it was all froze up the clutch never release the blades so they were always under full power. And let me tell you its loud, very loud. So loud it sounds like the motor is revving way faster than it really is. Once I got that corrected (and that's another story) the clutch releases with in 5 to 6 minutes and everything quiets right down. I also hooked a tach to see what the RPM were at the higher idle and it reads about 1100. I can live with that.

Dale

Thank you Dale for the info ! my homework confirms what you are saying about the higher than we like idle, I cringe when I start it on a cold morning. I only have 22,000 original miles on mine but I’m thinking about pulling that engine and going back in with a 580 HP, pump gas big block. Just tired of the TBI.

One other option you may want to consider would be an adjustable fuel pressure regulator.
I installed one in my 87 350 TBI. Has a hex key to adjust.
These units are set from the factory within a range and not necessarily tuned to the truck.

Thanks everyone for all the input !

Just my $.02 ... EVERY high idle speed I have dealt with on a TBI engine was from the base gasket. The high idle speed is caused by too much air getting into the engine. The ECM must then add fuel to keep the air/fuel ratio correct and that makes the engine speed up.