Surging Idle: Thinking out loud

[wczimmerman]

I know this is an oft discussed topic and I've done a fair amount of research already. I would like to jot down what I'm seeing in the hopes that I'm on the right track or maybe someone knows another thing to look at.

1987 V2500 5.7L TBI. Motor has about 28K miles on it (new block) with Edlebrock intake manifold. Fuel pump, fuel pressure regulator, injectors, fuel filter replaced already. IACV replaced with a Delco unit already. MAP and TPS also have been replaced. Wires/plugs are due soon. When I have the truck warmed up the idle does a steady up and down, like it's hunting for the correct idle speed. When in drive and warmed up: nothing...steady idle. When cold, starts up fine and runs well. There are no stumbling or acceleration issues warm or cold. If I coast at low speeds (less than 5mph) by letting my foot off the gas, you can feel a little bit of surging in the deceleration.

I plan to look for vacuum leaks, especially on the TBI gasket first and then around the other vacuum connections.

Any other ideas from folks?

 

[1987 GMC Jimmy]

I'd say definitely check for vacuum leaks. Also, make sure your timing is in the ballpark. Maybe see how 8* BTDC treats you. If you haven't done the IAC relearn procedure, you should do so. That's where you jump A and B on the ALDL for about thirty seconds with the key on, then unplug the IAC, turn the key off, disconnect the jumper, and reconnect the IAC in that order. I'm pretty sure that's how it goes. Double check me on that. I'd check your fuel pressure, too, just to make sure. Do you have any trouble codes? What's your EGR status with that intake and what modifications did you make if any? Did Holley/Weiand have any disclaimers for you when you bought that intake because if you didn't buy it from them, you should get the literature on it to make sure everything's right. I've looked at their products, and they tell you about things to modify and expect on their website.

 

[Rusty Nail]

I bet it's the ignition timing.

OR

Low coolant level.

OR both. Low coolant "washing" over the cylinder head sensor, retarding and advancing the timing UNTIL the thermostat opens.
Forcing into choke and out, changing the advance. This is EASY to see for yourself.

Verify the coolant temp sensor on the thermostat housing. There is a ground there but those wires/connections often fail.
Did you recently change the thermostat by chance? That sensor shorts internally and they are cheap.

Verify coolant level and follow proper "burping" procedure.
I'll bill half an hour if it doesn't cost me coolant , an hour if I have to add one drop - there's no other additional charge , plus I'm gonna fix it. If it's that sensor, cost becomes 50 bucks +1 hour.

-Coolant level or the thermostat housing sensor-

 

[austinado16]

I'm thinking vacuum leak through the EGR (partially stuck "open" at all times). Or vacuum leak via the brake vac booster or it's hose.

You can spray carb cleaner around the intake w/ the engine running, to look for intake manifold leaks. Any area you spray that results in a change in the engine rpm, is the location of a leak.

 

[wczimmerman]

Update:

I went out and first did the ALDL reset for the IACV. That made a BIG difference at the first start as the truck idled MUCH lower than before (not too low, it actually sounded perfect). However, when I touched the gas pedal ever so slightly it would do the engine surge while pressing the pedal. So, thinking that I maybe had a bad MAP sensor, too, I swapped it out since it had a "lifetime" warranty from the local auto parts store. Installed the new MAP sensor, fired it up, and still the idle hunt. Did the reset on the IACV again and it was again idling smooth again until I pressed the gas pedal. Driving it around seems fine.

One thing I did notice was that the IACV would vibrate or rattle when I went to unplug it with the key on (according to the reset procedure). I'm not sure if that is to be expected or a major clue that the IACV may be damaged or failed since I never did the reset when I installed it before. The rattle stopped as soon as it was unplugged.

I did check for vacuum leaks but I could probably do that again better. It was getting dark fast so I was trying to knock through things quickly. Nothing changed the idle one way or the other as I checked the TBI base and vacuum booster lines primarily.

Thermostat was changed with the engine 28K miles ago. Coolant levels are full. The temp sensor at the top of the intake was replaced some time ago with many of the other sensors.

The engine was professionally installed and running great when we picked it up (after a little fine tuning) 28K miles ago. On one of our family trips, it started doing this idle hunt and I'm finally trying to get it fixed.

 

[Georgeb]

The rattle from the IAC is normal. The next thing I would look at is the TPS. It may have a dead spot. Another culprit could be a worn throttle
Body shaft or the hole where it passes through the body.

 

[1987 GMC Jimmy]

Agreed. If there's play in there, you'll have a pretty decent vacuum leak, and you should get a bushing put in if it's bad enough. Would you be interested in seeing the live data? You could see what your TPS, MAP, and IAC (among other things) are doing so then there wouldn't be a doubt.

 

[wczimmerman]

Agreed. If there's play in there, you'll have a pretty decent vacuum leak, and you should get a bushing put in if it's bad enough. Would you be interested in seeing the live data? You could see what your TPS, MAP, and IAC (among other things) are doing so then there wouldn't be a doubt.

Yes, I would be interested. What do you have in mind?

 

[Georgeb]

You can make a cable and get software pretty easily....
http://www.gmsquarebody.com/forum/showthread.php?t=13194

 

[wczimmerman]

I bet it's the ignition timing.

OR

Low coolant level.

OR both. Low coolant "washing" over the cylinder head sensor, retarding and advancing the timing UNTIL the thermostat opens.
Forcing into choke and out, changing the advance. This is EASY to see for yourself.

Verify the coolant temp sensor on the thermostat housing. There is a ground there but those wires/connections often fail.
Did you recently change the thermostat by chance? That sensor shorts internally and they are cheap.

Verify coolant level and follow proper "burping" procedure.
I'll bill half an hour if it doesn't cost me coolant , an hour if I have to add one drop - there's no other additional charge , plus I'm gonna fix it. If it's that sensor, cost becomes 50 bucks +1 hour.

-Coolant level or the thermostat housing sensor-

Revisiting this again. I have replaced the TBI gasket and I tried driving it without the EGR vacuum solenoid plugged in. Without the solenoid, I would hear pinging when going up hills and my MPG was down. Plugged it back in and the pinging went away. At this point, I would like to know more about the burping of the coolant system since I have a 3 core Champion radiator on the truck which is VERY efficient. The thermostat was replaced with a 195F unit when the motor was replaced but the temp sensor in the intake was replaced before the motor swap and carried over. I could replace it but I didn't think it would cause a surging in park only. When the truck is in drive everything idles just fine...

I also plan to look at the timing soon.

 

[Rusty Nail]

I dont have anything else to add, I fixed this a long time ago.

EXCEPT!

Wait for it!


Y u no haz pix?


Only in park because rate of coolant flow = gph
Only in park because runaway idle speed.
Timing too advanced.

Your symptoms are resulting computer control and EPA gas mileage ******** not found PRE 81.

*edit*
You SHOULD be able to burp the coolant easily.
SHOULD be able to SEE air bubbles in the overflow. Add to correct level without removing the radiator cap, adding only to the overflow. Maybe squeeze the upper hose a bit - looking for bubbles in the overflow. It'll get HOT when the thermostat opens. Overflow should be at correct level. If bubbles remain present, the system is not sealed. Easy enough!

Got billed a whole nother hour for that.

 

[wczimmerman]

I dont have anything else to add, I fixed this a long time ago.

EXCEPT!

Wait for it!


Y u no haz pix?

Pix of what?

 

[Rusty Nail]

The truck?
Is it cool?

Thats what we do here, we like lookin at trucks...
You can tell a lot about a person by their truck.

[yt]egkrxkiUnoo[/yt]

 

[wczimmerman]

The truck?

Thats what we do here, we like lookin at trucks...

Gotcha. I will take some pics and get them posted.

 

[1987 GMC Jimmy]

You can tell a lot about a person by their truck.

Very true.