1988 R20 TBI High Idle?

[dwntwn]

I've had this Suburban for a couple of months and slowly fixing things on it. (1988, 454tbi, th400 trans, 3/4 ton)

It's always seemed to have a high idle, leading me to assume its got a vacuum leak. I replaced the intake gaskets (both on the spacer and the intake manifold), no change. I pinched the hose to the booster thinking it could be the booster diaphragm, no change.

Checked the vacuum hoses, some were old, replaced them all, no change.

Sprayed starting fluid all around the gaskets, intake, throttle body, didn't see an increase in rpms. Not 100% ruling it out, just wasn't able to identify a vacuum leak.

Pulled off the EGR to see if it was clogged (didn't look bad, wasn't stuck, seemed to hold vacuum) no change.

I'm not getting any codes, just seem to have a high idle. I don't have a factory service manual (yet) so i can't check the specs, figured i'd post and see if anyone had any similar experience.

Oh and i confirmed the timing is set at 4 degrees, timing bypassed. (Some people think it should be "0" ) but my core support label says 4 degrees, thats what its set at.

I put a tach in it and here's where the idle is:

• Cold start/fast idle: 1700rpm, moves up to 2100 rpm then after about a minute drops to 1200 rpm and stays there (vehicle in park, AC off)

• Fully warmed up and in gear: 850rpm

Sounds kinda high, right?

[R30 Casey]

I know this thread was started in 2017, Checking to see what the results were or if you solved the issue, I am having the same exact issues on my 88 R30, I am wondering what you did to resolve / fix your high idle issue. Any input from the user would be appreciated.

[chris989]

Originally Posted by R30 Casey

I know this thread was started in 2017, Checking to see what the results were or if you solved the issue, I am having the same exact issues on my 88 R30, I am wondering what you did to resolve / fix your high idle issue. Any input from the user would be appreciated.

Casey,
I know this is old and your post is a month ago,,,

Anywhoo,,, I chased a random high issue in my 5.7 tbi forever. I can data log and everything was alway in spec and then while driving it would idle crazy high until i turned the key off and restarted.

It turned out to be a defective TPS sensor. In the garage,, testing voltage it was always good. Even driving it would data log good. I finally caught it one day at a redlight. The TPS value spiked/jumped around and then went back to normal but when it failed for that few seconds it jacked up the computer without throughing a code.

Anyway, new TPS,, no more problem... I plagued me for years.

Need more help just ask,,, I will reply,

Chris

[R30 Casey]

Originally Posted by chris989

Casey,
I know this is old and your post is a month ago,,,

Anywhoo,,, I chased a random high issue in my 5.7 tbi forever. I can data log and everything was alway in spec and then while driving it would idle crazy high until i turned the key off and restarted.

It turned out to be a defective TPS sensor. In the garage,, testing voltage it was always good. Even driving it would data log good. I finally caught it one day at a redlight. The TPS value spiked/jumped around and then went back to normal but when it failed for that few seconds it jacked up the computer without throughing a code.

Anyway, new TPS,, no more problem... I plagued me for years.

Need more help just ask,,, I will reply,

Chris

Copy. I appreciate it. I have been chasing an idle gremlin for a while…