Strange idle and throttle while driving

Pat_Opsahl · Nov 3rd, 2009, 03:12 PM

Hello, new member here. Just acquired a 1987 HB truck, V6 with 5 speed and 180k miles. It came with this problem.

First the idle: When I start it cold the idle surges from near 0 to 2000 rpm. It will die if I take my foot off the throttle. As it warms up the surging shortens from 1500 to 2000 rpm. After complete warmup it idles at 1500 rpm.

Second the throttle response while driving. When driving, pushing the gas pedal will have no effect to accelerate the truck. It happens in every gear, and at random time intervals.
For example, I start to drive off from a stop sign and within 3 seconds the throttle does not respond, so I push the gas pedal harder, and harder, still no response. Then all of the sudden it goes, and it goes fast because I just about have the gas pedal floored because I was pushing it so hard. Shift to second, start to accelerate and again no throttle resonse. Same thing, maybe up to 5 seconds of no go and then it gets gas and goes? It does this all the time while driving.
It does not have the check engine light on. I checked all the vaccuum lines, found several that were hard and cracked - replaced with new, did not help.
I was not sure of the age of the gas so I added a can of heat to the gas tank, no difference.
When the engine is throttling correctly it feels good and strong.

If the check engine light is not on should I still check for codes?
My next thought is fuel delivery, fuel filter, check fuel pressure, check injectors.
The previous owner said it happened very ocassionly at first, once in a while and then slowly grew to all the time.
Any ideas and suggestions please,
Thanks, Patrick

white toy · Nov 3rd, 2009, 05:09 PM

Start by checking codes. You can have codes come up and the check engine light not be on.

pulsar86 · Nov 3rd, 2009, 09:58 PM

The bogging down and then accelerating only to bog down again could be caused by a faulty ignition coil. i had a similar symptoms once in a Datsun 1600 caused by a bad coil, check coil for correct resistance.

Pat_Opsahl · Nov 4th, 2009, 11:06 AM

One code came up: 13 - Engine Coolant Temperature Sensor
After checking the codes, I erased them and started and warmed up the truck. Let it sit and idle for 20 minutes to make sure it was at operating temperature. The temp gauge needle on the dash got to 1/4 on the scale, is this normal?

I then took off the air cleaner top and while it was idling sprayed throttle body fuel injector cleaner down it. The engine rpm would go up as I sprayed it. The more I sprayed in the higher the rpm's. This did not help, same high idle, 1500 rpm.

I looked at the coil located at the front of the left innner fender. I saw that the coil mounting bracket was cover with rust - could this be part of my problem? Remove the coil and completely cleaned off all rust from the bracket. Reassemble - still have the problem....

I started and ran it for another 20 minutes. The code 13 never came back?

Should I replace the engine coolant temperature sensor?

Thank for the responses, Patrick

Pat_Opsahl · Nov 17th, 2009, 01:12 PM

I found the problem!
When I was working on the truck, listening to the pulsing idle, it sounded like a fuel injector problem. With the air cleaner off I could see it flowing in gas without pressing the accelerator. I made up a patch cable to be able to disconnect each of the fuel injectors one at a time to diagnose them. I disconnected a fuel injector and started the motor - still have surging idle. Turned off, reconnected first fuel injector and dissconnected the second. No help. Disconnected both injectors to see if one or both were stuck open. The truck will not start - so, the injectors seem to check out.
Then I continued down this path, by disconnecting each of the fuel injection sensors one at a time and starting the motor. I was looking for a change in the surging idle.
I found it when I disconnect the air flow sensor. The surging stopped! When I went to reconnect the air flow sensor to verify my findings, I saw that the connector was very corroded. The pins had like battery post corrosion.
Cleaned the connector, connected the air flow sensor back and it idles normally!! Had to do some fine tuning with idle screw and wax idle assembly. Running normally for several days now, very happy. Thanks to all for your help!

pulsar86 · Nov 17th, 2009, 01:44 PM

Well done, you now qualify as a genuine engine detective.

zanegrey · Nov 20th, 2009, 09:29 AM

very cool...