Intermittent High Idle and Occasional Hesitation on Acceleration

[Moseph3]

I have an '87 R10 with a 305 in it, and a few months ago the truck would occasionally idle at a higher rpm. It would randomly do it for a while, and then it would go back to a normal idle other times I drove it. It's been idling high more often than not now, and sometimes it will start off normal and then idle high after driving for a while. I'm not very mechanically inclined, but have tried spraying the vacuum lines with carb cleaner but that didn't change the idle at all, so i don't really think its a vacuum leak. I'd also assume that if it was a vacuum leak it would idle high all the time no matter what, insteadof sometimes being high and sometimes being normal. I've also swapped out the IAC with a new one but it didn't fix it, it still sometimes idles normal but will also idle high other times.

I recently took it on a roadtrip and after filling up at a little corner gas station, I noticed a hesitation on acceleration a couple of times. There was no throttle response for a split second but then it would come right back, so I assumed that maybe the fuel wasn't the best quality. I put some Seafoam in it and ran through a couple of tanks of gas without any issue on the way home, but now that I've gotten back here it has hesitated a couple of times just driving around locally.

A few days ago I tried to do the idle reset procedure, I didn't do it when I swapped out the IAC. It didn't change anything at first, so I tried to do it again, and after that time it would rev way up as soon as I hit the key. I tried it again which didn't result in any change, so I threw my original IAC back in it and it returned to a normal idle. After driving it a few miles, it idled up while stopped at a traffic light and then hesitated when accelerating. It seems to be idling normal now more often than not though after swapping back to my original IAC.

Do you think the high idle and hesitation could be connected or separate issues? Has anyone ever experienced anything like this before or have any ideas on what the issue might be?

 

[DoubleDingo]

I'd say high idle is due to weather cooling down. But those with more experience and knowledge with these systems will chime in.

 

[Moseph3]

I appreciate the input, but it started doing it back in July when it was really hot out, so I don't think it's something weather related.

 

[gmbellew]

I'd bet that you are getting some sort of interference with the ICM harness or one of the other sensors wire harness is rubbed thru or something. it is throwing the ECM weird signals and causing the issue. check all the harness to the sensors and ICM really well, make sure plug wires are in good shape and not touching sensor wires, etc.

 

[YakkoWarner]

I don't know the answer - but I have similar symptoms. I can be sitting at an intersection and while waiting, the idle will creep up and then stays up until a restart. I can't find a vacuum leak either. I thought it might be the brake booster causing it while holding the brake at a light, so I did an experiment where I put it in gear with the wheels chocked and never touched the brake - still happens. I'm hoping that once I get the ALDL reader working I can find some clues....

I also get the hesitation when starting off. Not even sure where to start on that one, again hoping a data capture will shed light on things...

 

[gmbellew]

I don't know the answer - but I have similar symptoms. I can be sitting at an intersection and while waiting, the idle will creep up and then stays up until a restart. I can't find a vacuum leak either. I thought it might be the brake booster causing it while holding the brake at a light, so I did an experiment where I put it in gear with the wheels chocked and never touched the brake - still happens. I'm hoping that once I get the ALDL reader working I can find some clues....

I also get the hesitation when starting off. Not even sure where to start on that one, again hoping a data capture will shed light on things...

If you have the wrong type of EGR valve , it will hesitate upon light acceleration from a stop. Might not be the issue you have, but it definitely does it with the wrong valve

If the RPMs in the data stream are jumping all over at readings that are no way correct, it is some sort of interference in the ICM harness most likely.

 

[Moseph3]

I'd bet that you are getting some sort of interference with the ICM harness or one of the other sensors wire harness is rubbed thru or something. it is throwing the ECM weird signals and causing the issue. check all the harness to the sensors and ICM really well, make sure plug wires are in good shape and not touching sensor wires, etc.

Thank you, I'll have to.look into that

 

[Jgonick]

I don't know if this fits here... But i found this video interesting- in case you have a failing/failed ESC. Video shows a simple way to bypass (temporarily) while not messing up wiring.

How to Bypass ESC Easy -LINK-

Edited--

Not applicable to 87. Different ignition system.

 

[Ricko1966]

I don't know if this fits here... But i found this video interesting- in case you have a failing/failed ESC. Video shows a simple way to bypass (temporarily) while not messing up wiring.

How to Bypass ESC Easy -LINK-

Not applicable to 87. Different ignition system. Hesitation I like to look at the TPS.

 

[Poppy 87]

My 87 had similar issue, found worn throttle shaft bushings in throttle body. Caused throttle plates contacting throttle body as well as vacuum leak. Inspect inside throttle bore while moving cable end of throttle shaft. There should be no contact between plates and housing, nor any lateral movement of throttle shaft. Good luck

 

[chris3353]

Hello,

A TBI? yes?

Please download WINaldl. It is free, you can make or buy a cable for ~20$. Without it, you are wasting time and $.

It will tell you every flag/error.

Example, common issues;

Truck does not know what gear it is in.
TBI sender thinks engine is cold.
Could be calling for AC when not present or off.

There are a handfull of trigger for idle. I can easily help you if you collect the data. A TBI motor should idle at ~550 stock. I program you a chip if needed.

Cheers,
Chris

 

[Ricko1966]

Hello,

A TBI? yes?

Please download WINaldl. It is free, you can make or buy a cable for ~20$. Without it, you are wasting time and $.

It will tell you every flag/error.

Example, common issues;

Truck does not know what gear it is in.
TBI sender thinks engine is cold.
Could be calling for AC when not present or off.

There are a handfull of trigger for idle. I can easily help you if you collect the data. A TBI motor should idle at ~550 stock. I program you a chip if needed.

Cheers,
Chris

Tons of people on this forumn have been trying to get cables,only supplier I've been able to find that actually has,is still in business,and will ship ALDL cables is moates and their cable is 80.00 Do you have a secret source for 20.00 cables or instructions in how to make one. Instructions that are not 20 years old that require obtaining components that were easy to get 20 years ago but not now, If so that would be really helpful to some members. Only instructions I've seen include what radio shack part numbers to buy. Radio Shack is gone so their part numbers do no good. Thank you sir.

 

[seabee]

I have that same year truck and engine. Owned it since it was new . With that being said I had a bad EGR valve and it caused hesitation when I would leave my yard on a small incline. It would almost stall out. Would start checking EGR valve. Thanks

 

[Fat 454]

Tons of people on this forumn have been trying to get cables,only supplier I've been able to find that actually has,is still in business,and will ship ALDL cables is moates and their cable is 80.00 Do you have a secret source for 20.00 cables or instructions in how to make one. Instructions that are not 20 years old that require obtaining components that were easy to get 20 years ago but not now, If so that would be really helpful to some members. Only instructions I've seen include what radio shack part numbers to buy. Radio Shack is gone so their part numbers do no good. Thank you sir.

Seconded - not to hijack the OP, but is there an ALDL thread / sticky on here that actually shows what is needed / how/where to plug in, required cable ends, software / scanner suppliers, years affected ( GM ALDL v carb v OBD 2 ) etc. ? Agree this would help a lot of people here.

If not maybe we should start one ?? ..

 

[YakkoWarner]

I started a thread about it to discuss my own experiences (which right now involves waiting for a battery so my laptop will finish starting up before dying) - feel free to hijack it all you want with regards to ALDL, the more we know the better off we all are!!

https://www.gmsquarebody.com/threads/aldl-data-logging.48253/