Erratic Idle

[Rod_Lei]

So I changed out a bunch of sensors on my 88 5.7 350 and got the engine to idle way down. For like a week it ran very well, AutoIdle worked in the mornings, got nice and low once it got warm. Well recently, I've had this spontaneous Idle change everytime I drive the truck.

I'll start it up, it will idle up, hit operating temp, then come down to normal idle. Well I'll be driving for a little, and suddenly the idle will go way up and stay up no matter what I do. But. If I turn the truck off and start it back up, it will be ok for a while.

Does anybody have any idea what would cause it to jump up the idle like that? when it has high idle, it runs normally just like it would when it is cold, I think it's a sensor, but I don't know which one would cause such a high jump. Any help would be awesome!

[JWall]

Throttle Position Sensor?

[Rod_Lei]

I already changed it:/

[Rod_Lei]

I've changed the Idle Air Control, the O2 Sensor, the Throttle Position Sensor, the EGR Solenoid, the Coolant Temperature Sensor. Could the MAP Sensor cause this? The idle rise is not gradual, it just suddenly goes up and stays there, no hunting, just a steady high idle.

[SiCk_DiAbLo]

Did ya unhook the battery cable?..Maybe it'll reset!!!

[70BLAZERX2]

Worn distributor shaft ..you will have to pull it out and try to turn it...if its tight..that will be it..very common for these to do that. They will start to sieze up and throw the ecm in a fit.

pickup coil fires plugs and also injectors from the rpm signal in the dist.

Make sure to unplug the tan/white single wire to set timing back to "0" degrees

also pull the throttle body and clean all passages...this will cause a vacumme leak if plugged
it will open the adle air motor wide open.

[Rod_Lei]

Well if the Dist was bad wouldn't it be a timing problem? Like the idle goes up, but the truck runs fine, just with a high idle.

But how do I check if the IAC is getting pulled open by vacuum?

[70BLAZERX2]

The iac is electric controlled by computer. it creates a vacume leak for idle..

I had a 87 burb 350 do the same thing..changed everything ...it would do fast idle..then idle down...as you drove it..fast idle would come back..changed the dist..problem gone...

years later...3 more trucks i worked on did the same thing...And its not that hard to change.

Been wrenching ever since 1987

[Rod_Lei]

Ok, so we're going to pull the Throttle Body off and clean it out. It looks like 3 bolts hold him in there? Is there anything I need to watch out for or something I should go ahead and change while the Throttle Body is off?

And how hard is it to pull the dist out? Will I have to go through the timing nightmare?

Sorry for all the questions, I've done work with engines but never on these 350's, and they run like monsters but seem to be whiners if you mess something up...

[70BLAZERX2]

Just 3 bolts..16mm and 19mm on the fuel lines..make sure you do not loose the little o-rings on the lines..if you do you will need to get them new.

The bottom of tbi unit will have grooves in it for vacume ..just clean them out good with carb cleaner..try to keep cleaner away from the throttle pos. sensor..

remove the IAC and clean it to...its the has big nut on the side. If these are stopped up..will create problems for the map sensor and ecm will do crazy things..

I also worked on a fleet of taxi's a few years back..all 350 91-96 caprices..they had the same problems too.

Could even be the computer itself. if you know someone who has one,change it and see if problem goes away.

Just because the check engine light is not on..dont mean that it is right

If your check engine light is on with key on engine off. it is ok
If it does not come on..it is bad or bulb is out in dash.

[ChevyTech]

I think you should diagnose the problem instead of taking the TBI apart which may result in errors that will complicate getting it to run correctly.

Do NOT get carb cleaner on the TPS or in the IAC.

Have you checked the ECM for trouble codes?

I agree with 70Blazerx2 that the distributor could be a problem:

On these distributors the housing/upper bushing area can get so worn out that the shaft will move sideways so far that the points on the pickup coil pole piece get hit by the reluctor points on the shaft. When this happens the ECM receives compromised signal for RPM and timing issues.

Did you check the distributor for shaft side play yet?

You should be testing for poor grounds. People are so bad at testing grounds I suggest taking a jumper cable and connect one end to the engine and the other end to bare metal on the firewall, then the frame.

The TPS ground terminates at the front of the intake manifold and all the grounds should be taken off and cleaned and reinstalled.

The ECM controls the idle speed through the IAC. Did you inspect the wires to and terminal ends in the connector for the IAC?

The IAC is a stepper motor and uses short pulses of electricity on the wires to move its position.

If the IAC sticks as does not move as it should for each pulse, the IAC count will get off because the computer does not know the actual position of the IAC, but the ECM tracks the commands it has sent to the IAC and assumes it has moved every time it has been sent a pulse.

Does the truck have an automatic or a manual transmission?

How well does the IAC give it a fast idle when started and how well does it smoothly slow the idle down after as the engine runs after it is started?

[bwood]

Could it possibly be a vacuum leak between the EGR solenoid and/or the EGR valve. This might explain the "all of sudden" high idle?

[70BLAZERX2]

this is one of those ,cannot fix it over the computer things...LOL

Without being there..Post a pic of inside of distributor..I bet its all green around the pickup coil..

Yes dont take tbi apart. just clean it out...those injectors are very pricey..

[Rod_Lei]

Ok Ok lots of questions. I already changed the IAC because it was sticking, it raises the idle when it's cold and smoothly drops down, there is no sudden drop or hesitation. It is not even a month old.

The ECM has no codes, the truck hasn't given me a Check Engine light. I checked it and got only 12, the Diagnostic Code.

I checked the Dist if it had any play and it is solid. The Cap has all even wear, that's why I doubt a problem with the Distributor. I cleaned the Dist out when I changed the Ignition Control Module, so the green is gone:P

The truck is a 1988 5.7 350 Automatic.

My friend mentioned that it could all be a severe vacuum leak, I've noticed something; If I hit the brakes, it will rev up, and when I let go it also revs up. It is much more noticeable before I start driving it and am about to put it into gear. It will be idling, I'll hit the brake to shift, and it will rev way up. once it's warm, it will rev when I press the brake and when I let go. I don't know if this is a problem with the Booster or if it is a symptom of some distant vacuum problem elsewhere in the truck:P

Thanks a lot for the replies guys!!

[Rod_Lei]

Oh! and I haven't checked grounds yet, I'll check it once it starts acting up tomorrow!

[Zeke]

A leaking brake booster would be a major vacuum leak and will cause the ecm add more fuel to correct the lean condition. You can drive with the booster disconnected and plug the port on the TB, just remember you will need a lot more force to brake.

Also you can check the booster and check valve by shutting off the engine without stepping on the brake, you should have 3+ pumps of assist with engine off, if it doesn't change the check valve first (cheapest).

[ChevyTech]

It sounds like you have a bad brake booster.

When it is idling to fast, pinch off the vacuum hose to the booster and see if it slows down. If it does the booster has a torn diaphragm in it.

[Rod_Lei]

Ok I'll try that! For some reason it doesn't want to act up today, I've driven it all day and it doesn't want to do the usual high idle, now it just has no power.

[ChevyTech]

Keep us up to date when it acts up and you get to test it.

[Rod_Lei]

Well the truck doesn't want to do it's idle problem, now it idles low, kind of choppy, and it just has no power. I tried pinching off the Booster, nothing. I'm just gonna keep driving it till whatever it is causing my problems decides to just quit. Once the weather warms up I'll test the Dist again. Thanks a lot for the replies guys! I'll let it get worse and maybe it will be easier to diagnose:P

[Rod_Lei]

Alright so we unplugged the booster today, truck ran like a champ. So the booster is now on my To Buy list. Can anybody give me the rundown on how to remove and replace the booster? Anything I should look out for?

Thanks in advance!

[Boog]

You can replace the booster without taking the brake lines loose from the master cylinder. Remove the bolts holding the master to the booster and slide the master forward enough to clear the booster. The brake lines will bend enough to allow this. Go inside the truck and remove the wierd clip holding the push rod to the brake pedal. You will just have to use a long screwdriver or similar tool to pry that clip off. You will most likely destroy the clip. They are available at GM dealers for about $2.50 or so and some parts stores carry them in the Dorman line. You would think a new clip would come with a replacement booster but they don't. Remove four nuts inside to loosen the booster from the firewall and pull it out. Reverse to install. It's a tight fit up in there. I use a magnet stick to hold the new clip in place while I push it over the pin.

[ChevyTech]

In addition to what Boog said, the brake light switch may get moved out of adjustment when swapping the booster.

Here is how the switch is adjusted:
On the curved side of the switch, which is pointing down and somewhat towards the front of the vehicle when installed, the end of a lever protrudes slightly out of a slot.
While holding the brake pedal down a little ways, slide the lever that protrudes from the slot toward the driver’s seat.
Let go of the lever, and then return the brake pedal to its fully released position.

[Rod_Lei]

Thanks guys! Yeah I was wondering what to do with the various electronics hanging around the pedal. So the clip, I looked at it today, it looks complex and weird. Do I just take the screwdriver to it and jerk it out with some leverage? Or is there some way to save the poor thing?

[Boog]

No jerk to it. Just a twist of the driver to pry the clip off. Yeah, it's frustrating.