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Low Idle RPM's - Yet Not Adjustable?? Per the VP shop manual?

Chris Rohde

Regular Contributor
1995 5.8FSiPHUCCE

My idle RPM's per the book should be 600. Right now, I'm closer to 500, and sometimes it will dip to around 400. It's not steady, and it's not as high as it should be. It's a bit rough, and I would really like to get it back to the steady 600 it should be. Yet, the book states "Not adjustable". What am I missing here? It seems there has to be some way to adjust this. Clearly it's not in adjustment now.

Any ideas?
 
Based upon some reading on other forums, I'm going to go ahead and swap out the Idle Air Control valve. It's a $50 part that takes 5 mins to swap out. Mine is the original 1994 part, so even if that does not remedy the situation, I won't feel bad about replacing something that old. I'm also going to look at fuel pressure. Any other thoughts or input would be greatly appreciated.
 
Not sure. That's what I always did on my old carbed Chevy. Was just a bit surprised by the "Not Adjustable" in the book. I can try the cable, but I don't know if that will help with the wandering RPM's. Thatought maybe the IAC would help with that part.
 
Not sure. That's what I always did on my old carbed Chevy. Was just a bit surprised by the "Not Adjustable" in the book. I can try the cable, but I don't know if that will help with the wandering RPM's. Thatought maybe the IAC would help with that part.

Don't touch the cable.

The computer maintains Idle with a controlled vacuum leak through the IAC motor. If you adjust the throttle cable you will throw off the TPS and you will make it worse.

On a Fuel injected engine that is not idling correctly start with the IAC then check the condition of the dist cap, wires and plugs. If all is good, and the idle is still an issue, time for a cylinder drop test followed by a compression test.
 
That has the TFI (thick film ignition) so ignition timing is completely controlled by the computer as well as the injector system. All of the inputs are very important for proper computer function. Go through and clean all of the connectors on the engine. Pull them apart and clean with electrical cleaner and once dry put back together with dielectric grease. Sand all grounds metal to metal. Not sure if you can use a OBD II type scanner to reset the computer, on ford cars you need to reset the computer any time you change or clean a senser.
 
Thanks for all the suggestions guys! Plug, cap, rotor, and coil are new. Plug wires are not. IAC is in the mail. Never thought about the PCV Valve. Scanner shows no codes.
 
To check ignition timing you just disconnect the spout shorting bar. Right next to the plug going to the ignition module is a two wire connector with a pull out shorting bar. Bring the engine up to normal temp and at idle speed pull out the shorting bar and it puts the ignition at base timing. then you can adjust the timing if necessary.
 
Just went out in the garage and looked. Not on this one. Doesn't look like there is one on these valve covers for the GT40 heads.
 
I'm also going to run some Sea Foam through the fuel system. Unfortunately, I only go out a few times a year, and I'm more often than not running on old fuel that I NEVER treat. It eventually gets mixed with fresh, but I doubt I fuel up 3 times a year. That's a lot of sitting fuel. (i do change the fuel filter every year though).I can definitely see how that could contribute to a low and rough idle. So this week will be new IAC, New plug wires, and some Sea Foam. Hope those do the trick.
 
Back to the PCV, I vent directly below the oil fill cap, right back up into the flame arrestor. 100% confident that I don't have a PCV valve.
 
Well, just as an update, my new IAC valve took me from a bouncing 400-500 rpm, to a very solid 1,000 rpm which is now higher than the 600 the book says I should be at. I'll recheck timing tomorrow.
 
Well, just as an update, my new IAC valve took me from a bouncing 400-500 rpm, to a very solid 1,000 rpm which is now higher than the 600 the book says I should be at. I'll recheck timing tomorrow.
I believe you need to run it to let the iac motor adjust. For cars it recommends you run it at least to 40 mph so run it hard for a bit then recheck the idle.
 
I believe you need to run it to let the iac motor adjust. For cars it recommends you run it at least to 40 mph so run it hard for a bit then recheck the idle.

Great point. Taking the boat out on the 5th and 6th after all the loonies leave the lake. Will run it up and down the lake a number of times and we'll see what she does.
 
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