Sudden engine revs

[Jehu]

1993 Chevrolet K1500 5-speed manual, 5.7L TBI

Last night, I was just taking off from a light and the engine just started red-lining. I tapped the gas pedal thinking that it was just caught on a floor mat or something. The pedal felt normal. After 2 or 3 seconds, the engine went back to normal speed.

It happened again this morning. This time, I was between 2nd and 3rd gears. It just surged. I hit the clutch and was ready to kill the engine when it calmed back down.

Any ideas on what could cause that?

 

[DerekTheGreat]

Unplug your IAC valve and see if it still does that. I'd do the thing where you connect the A&B terminals on the OBDI port (The top two on the right of the plug). What you do is connect the two with the key off, then flip the key to run and wait about 30 seconds. The IAC valve should be closed at this point so disconnect it and then flip the key off. Pull the jumper from the A&B terminals and then drive around trying to repeat the condition.

 

[DerekTheGreat]

I've been having all sorts of weirdness with mine but I think it's just how the truck wants to be. I'm on my third IAC valve and despite cleaning the entire TB it has made no difference. With the IAC closed like I described, truck acts normal and the RPM's come down fine inbetween shifts. With the IAC connected it will try and hold ~2k or 1,500 RPMS for awhile before slowly coming down.

 

[Jehu]

Thank you, Derek. Is there a chance that a defective IAC valve can cause engine miss?

 

[DerekTheGreat]

You're welcome.

I'm no professional but I'd say no. All the IAC valve does is let extra air in around the throttle blades to better regulate idle. A miss would be something else like from bad plugs, wires, timing, stuff like that.

Does it miss all the time or only under certain circumstances?

 

[Jehu]

I have another thread going about this. I just got the truck a couple of weeks ago. It runs rather rough. It's real easy to stall. I need to feather the clutch. Aside from that, when it's up to speed, it's got a definite miss going. Nothing real extreme, just annoying.

I've changed plugs, wires, cap, rotor, temp sensor and MAP sensor. I'm wondering where to go next. I was getting codes 42 and 54. I tried cleaning the ground wires that attached to the thermostat housing. I got noticeable improvement after the plugs and wires but the miss is still there.

 

[Jehu]

... I cleaned the ground wires at the thermostat housing a couple days ago. I just now checked engine codes. It just repeats 1 & 2 so I guess all codes cleared up. Now I'm clueless on what may be causing the engine miss.

 

[Justin S]

My old 94 tbi had a sticky throttle, once every 6 months id go to leave a stop sign and id press the gas and it would stick, id have to get out and play with it back and forth a few times and it would free up. Never figured out what was sticking, I just kept lubing everything, but it would still do it

 

[Justin S]

Also try the $10 fuel filter mod :-D

 

[DerekTheGreat]

Well if you've covered the basics it might be time for a compression test.
I had a sticky throttle too, had to lube up the TB and it's return springs as well as replace the throttle cable.

Sent while eating biscuits and French-fried taters mm-hmm.