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1990 suburban with high idle ?

S

sidewinder

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Hello,I have just recently purchased a 90 sub with a rebuilt 350 tbi.I dont have much history on the truck as the sale is due to a divorce.The motor seems to have a very high idle which I dont seem to know how to get down.I have tried the throttle adjustment on the tbi and timing with no luck.I also sprayed carb cleaner at the base to check the gasket with no change in rpm.It does have an open chrome air cleaner on it so there is no heat going to the intake,not sure if this would have anything to do with it but even on 60 deg. days driving in city conditions the rpm stays high.It does have a very loud sucking sound at the tbi. Which does sound like it is the air going into the top of the tbi.Any suggestions?TIA.
 
I am unsure about this but you can check it out as I do not have TBI vehicle. If it has a throttle positioning sensor on it it could be bad. My fiancee's 95 Jeep GC had a high idle, about 2000 rpms, and I ran a scanner on it and it said the TPS was bad, changed it and it solved the problem. Anyone that has TBI probably knows if it has one.
 
Could be the Idle Air Control is out of whack! If you've messed with the Torx head Minimum Air screw that could be off too..compounding the problem!!
Also check ESC and TPS!
 
I will try to test the tps today.I did try the torx screw on the drivers side of the tbi.That did not change a thing.The flaps in the tbi seem to be staying pretty much closed.I will also try to test the Idle air control today.Thanks for the suggestions.I will give an update once I get these items tested.
 
Try cleaning your IAC (Idle Air Control valve) Remove it and spray some carb cleaning into the opening and let it sit a while. If the old valve is nonop do your self a favor and get a REAL OEM valve from GM. I had a problem with an aftermarket valave.

goodluck

Dan..
 
Change the gasket anyway. I had the same problem with my 454. It cost me $3.00 and only took a few minutes.
 
Well,I tested the tps which was testing o.k.,Then it was onto the Iac.That tested out fine and was very clean inside.I tried to clean both of these items out.I then went and picked up a scan tool to check for codes.It gave me 2 of which were from different tests I did to try and eliminate poss.Then I cleared the codes and took the truck for a spin to see if anymore codes poppped up,but it still shown no codes stored.I also tried to get the gasket with no luck,it is on order.I did spray some carb cleaner around the tbi to see if there was any change in idle.There was none other than when the fumes went in at the top.I also noticed that at some points it does seem to idle fine.There is no massive vacuum noise from the tbi.If the air cleaner is off and I place a rag over the back half of the tbi the noise goes down a lot but not the rpms.I dont see anything out of the ordinary back there.Now just another question.I do not know what the specs are on the motor itself.So if this had something like an rv cam could this be causing this problem?When first started you can hear what sounds like the thump of a cammed motor.It goes away with in the first 20 secs. or so.Could this be the computer trying to over ride the cam??Thanks for all the help so far.Rich K.
 
I also sprayed carb cleaner at the base to check the gasket with no change in rpm
Click to expand...

Neither did mine. I still had a gasket leak.


Change the tbi gaskets. And check all the vacume hoses. The high TBI idle in both my trucks were vacume leaks. I was getting engine lights too. Both are fixed now. Change them, go.. ;)
 
tomseviltwin said:
Neither did mine. I still had a gasket leak.


Change the tbi gaskets. And check all the vacume hoses. The high TBI idle in both my trucks were vacume leaks. I was getting engine lights too. Both are fixed now. Change them, go.. ;)
Click to expand...


What Tomseviltwin said!

After finding a leaky vacuum port, idle dropped a little bit. After changing intake to spacer and TBI base gaskets, idle is now normal.
 
wouldnt explain the air sucking in, but what about the coolant temp sensor ( I think thats what its called), the sensor is at the front of the intake if I remember right. That happened on a suburban I had. It would idle real high, couldnt get it to come down. Replaced that and it ran like a champ. Basically it thinks the engine is still cold even when its not so its like having the choke on all the time.
 
I have the gasket on order,It should be there but I prob. wont be able to get it until this weekend(14 hour work days+1 hour each way to get there).The coolant temp sensor could also be worth trying.It does act like the choke is on and just wont come down.That will make another thing to check this week(end).Thanks for the ideas,I will give an update when I can get to these things.
 
If that doesn't fix it you might try checking your knock sensor on the right side of the motor near the starter, my '90 k5 idled too high till I put in a new knock sensor, but was getting a dtc 43 under hard acceleration!! Just a thought.... don't know if this helps!!!

BIGG R
 
Well,I replaced the gasket with no change in the idle.Then I changed the thermo.(180 to 195)with a little change in the idle but not much and as soon as I tapped the gas it went right back up.I wanted to try and change out the temp. sensor but I forgot to pick one up.Is there a way to test the coolant sensor/the knock sensor ?Thanks Rich K.
 
I checked mine with a Tech 1A scanner! I don't recall any shadetree mechanic ways of checking the operation of the sensors off hand, but I'll check my service manual and see if they can be checked with a V/O meter to see if their reading right!!!
 
I checked the system with a scanner.It was one of those small ones that plug in under the dash and will flash the engine light.That did not pull up any problem codes.I dont know if that is the same type of scanner that you are refering to.Thanks,Rich K.
 
You may have a pinched wire in the harness that drops down towards the tranny behind the dist. It is a big harness. That happened to the 90 burban I just bought.
 
You can test the temp senders (or any sender) but it wouldn't do you much good since you don't know what the actual engine temp is. Typically they will be WAY off (like max or minimum that they can register) when they go bad.

Somewhere I believe there is a post with the temperature/ohm scale.
 
TPS should be set at .6-.65 volts. It could also be a vaccum leak on the intake manifold. There is a procedure to reset your IAC.

Courtesy of SouthernSpeed:

You need to put the jump wire between A&B on the aldl socket. Turn ign on but don't start it. Wait 30 secs then unplug the IAC plug. It's the square plug on the right hand side of the throttle body (as you sit in truck). It sticks out and backwards a bit, not the vertical one just in front of it (TPS). Take jumper wire out, start engine and set the idle speed. Switch off, replace plug and disconnect battery for 1 min. Run and check idle!
 
If it isn't throwing any codes you might check for a vacuum leak or the timing might be too high. To check the timing, unplug the bypass plug located near the bulkhead for fuse box(engine compartment) should be a tan wire w/black stripe, unplug this and set timing to 0 deg with a timing light!!!
 
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