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1993 4.0 low idle, wavers up and down


EHeaney

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Joined
Sep 25, 2016
Messages
4
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Location
Sac, CA
Vehicle Year
1993
Make / Model
Ford
Transmission
Manual
Alright so as the title says, my truck idles pretty low once warmed up, and it wavers up and down about 100 rpm. It's especially bad with the A/C on, sometimes it drops under 500 rpm and the truck shudders a bit. Here's a vid, vertical vid warning, my bad:

https://www.youtube.com/watch?v=LWYOgOAcxPA&fs=1" width="644" height="390">https://www.youtube.com/watch?v=LWYOgOAcxPA&fs=1" />https://www.youtube.com/watch?v=LWYOgOAcxPA">http://www.youtube.com/watch?v=https://www.youtube.com/watch?v=LWYOgOAcxPA

Direct Link: https://www.youtube.com/watch?v=LWYOgOAcxPA

Manual transmission so no torque converter load. A few seconds in, I kick on the A/C and you can see how much it dips. Can't tell on the vid but when it's at it's lowest speed, the engine shudders. I revved it up and held it and it has no problem holding a certain speed with the throttle. Also it drives perfectly fine. Just seems to struggle at idle. Which is a problem because it's overheating on 100+ degree days when idling with the A/C on, and I think it's because there's not enough fan blowing/water flowing at such a low speed with the A/C load.

I replaced the IAC and PCV valve, and I've been searching for a vaccuum leak and can't find one, though I wouldn't be surprised one bit to find out I just missed it. I'm wondering what you guys think should be the next culprit.

I can't tell if this is a mechanical failure such as a vaccum leak or sticky valve somewhere or if it's more of an electrical/sensor failure, as in something is telling the IAC to open, close, open, close, open, etc., though this could also be caused by some mechanical failure I guess.
 


Rock Auto 5% Discount Code: F9A1A579ACFAD1: October 1st, 2021

RayInStl

Member
Supporting Member
Joined
Oct 30, 2015
Messages
267
Reaction score
1
Points
18
Location
St. Louis, MO
Vehicle Year
1983
Make / Model
Ford
Engine Size
2.3L
Transmission
Manual
My credo
Well, that didn't work. Let's try something else!
That's classic vacuum leak. Your A/C puts load on the engine, so it's not surprising that it gets worse. Your A/C controls are vacuum operated, also. I suggest getting a can of starting fluid and spraying likely leak spots (hoses, intake at the gaskets, brake booster, etc. If the engine revs, you found your leak.

Good luck!
 


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