Engine won't run without throttle help on startup


Evan

Evan

15+ Year Member

Joined
Aug 11, 2007
Messages
1,373
Points
3,101
City
Stevens Point, WI
Vehicle Year
1991
Transmission
Manual
When I start my engine, it starts and then several seconds later will stall unless I get on the gas. When it warms up, it will run on it's own but the RPM is real low (300-500).

Could this be the IAC? I cleaned it, but maybe it's still not functioning?
 
I would suspect the IAC, when you cleaned it, did the plunger move in and out freely? Maybe she's stuck, and not opening to get the idle up.
 
BlackBII said:
I would suspect the IAC, when you cleaned it, did the plunger move in and out freely? Maybe she's stuck, and not opening to get the idle up.
Click to expand...


Yes, the plunger moved freely.

When I shook the solenoid I could hear the steel shaft going from side to side. I don't think anything is gummed up, but maybe the coil just failed?
 
That could be, I would take it off but leave the wires plugged in, start the truck (cover the hole, or you will idle like a son-of-a) and see if it moves at all.
 
If your EGR is clogged it will do the same thing from my previous experience. Just a thought
 
I don't have EGR on the engine. I just ordered a new IAC for $39 on Ebay.

Also, when started the engine misses for a few seconds as I give it gas to keep it alive. Then it smoothes out.

I have the throttle adjustment screw cranked in to give the engine enough gas, while I determine what the problem is. Hopefully a new IAC will solve it.
 
That just screwed up your TPS adjustment. Bad move.

Might try priming the FP several times before starting it (key to Run, pause pause while FP runs then shuts off, key to Off). Repeat.
 
Test your CTS and ACT sensors at the PCM connector.

Check to see if the FPR holds vacuum.

And look for vacuum leaks.

For whatever reason, the mixture is off. This makes it much more difficult to run cold.
 
MAKG said:
Test your CTS and ACT sensors at the PCM connector.

Check to see if the FPR holds vacuum.

And look for vacuum leaks.

For whatever reason, the mixture is off. This makes it much more difficult to run cold.
Click to expand...

I have replaced the CTS, but that doesn't mean the signal is getting to the PCM I guess. A bad air-charge sensor is possible.

How do I test those sensors at the PCM? I hate to admit it, but I have very little experience diagnosing EEC-IV systems.

I did not check the vacuum at the FPR because I don't have a gauge, but I did take off the line and sniff it for gas fumes, and there were none.
 
Earl43P said:
That just screwed up your TPS adjustment. Bad move.

Might try priming the FP several times before starting it (key to Run, pause pause while FP runs then shuts off, key to Off). Repeat.
Click to expand...


It was the only way I could get it to idle well; I had to get to work. Can't I just back the screw out again and the TPS will readjust? I guess I don't understand what the difference is between cranking in that screw and manually giving it gas via the pedal.

Once warmed up, the engine idles and drives real smooth (with that screw holding open the throttle a bit).
 
I bought a code reader and did a KOEO test. Here's what I got:

85 - Canister purge circuit failure (expected)
29 - Insufficient input from vehicle speed sensor (expected)
41 - No HO2S switching detected (???)

EDIT, just replaced the O2 sensor, no improvement.
 
Last edited:
evan, i bet its the upper intake manifold gasket. might be brittle or cracking and once the engine warms up the gap or gaps fill in enough to not leak
 
dgndngrngr said:
evan, i bet its the upper intake manifold gasket. might be brittle or cracking and once the engine warms up the gap or gaps fill in enough to not leak
Click to expand...


Upper intake gasket is brand new. I replaced it when I replaced the pushrods and rocker arms. Bolts are torqued to spec.

A little background on the engine:

2 years ago I did a full rebuild. This included a hot-tanked, leak-tested, honed block, new rods, pistons, rings, seals, bearings, gaskets, fuel injectors. Heads were checked out by a machine shop, cleaned and deamed re-usable.

Recently replaced: Pushrods, rockers, O2 sensor, coolant temp sensor.

IAC has been cleaned and seems ok. Plug wires, plugs are new.
 
when I took off my iac I just sprayed throttle bottle cleaner into the two holes. How do you tell if that rod with the spring on it moves freely? I did not see anywhere to grab ahold of it to move it back and forth. Is there a way you can dissasemble it more to clean it better? Also what is the black circle knob that threads in and out for? this is on a 2000 ohv 4.0
 
truckingixxer said:
when I took off my iac I just sprayed throttle bottle cleaner into the two holes. How do you tell if that rod with the spring on it moves freely? I did not see anywhere to grab ahold of it to move it back and forth. Is there a way you can dissasemble it more to clean it better? Also what is the black circle knob that threads in and out for? this is on a 2000 ohv 4.0
Click to expand...


I don't know if we have the same IAC. But I removed the two screws that hold the solanoid to the plunger chamber and separated them. I was able to clean the plunger chamber better that way and move the plunger back and forth.

I got my new IAC in the mail today. I have a feeling it won't solve the problem, but it's worth a shot I guess. Would a bad air-charge sensor throw a code?
 
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