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Truck hunts, lopes at idle

Tiny
DJKENNY619
  • MEMBER
  • 1989 FORD F-250
  • 5.0L
  • V8
  • 4WD
  • AUTOMATIC
  • 150,000 MILES
Cold, starts up nicely, smooth, fast idle. Then as warm up progresses, the idle will slow to "normal". Then the loping starts, mildly then to a almost stall to a fast rev much like when cold then, stall. Restart, no issue, "normal" operating temp idle but after 2-ish minutes it start the lope.
Replaced:

Throttle body (a shop told us they did a smoke test on it and it leaked).
Throttle position sensor.
EGR valve.
EGR position sensor.
iac (idle air control) valve.
Engine vacuum at 15/5hg at idle, needle smooth.
Fuel pressure, 42.5 key on engine off.
Fuel pressure, 35 key on engine running.
Fuel pressure regulator disconnected 55 (if my memory is right) engine running.

Based on all the internet info I could find with the fuel pressure seems to be within acceptable range.
Removed the duel air intake tubes, choked the air horns, engine stall, indicating no vacuum leak.
Excluding the MAP vacuum hose all other vacuum hoses disconnected and plugged.
Still the warm engine lope exists.
Unplugged the SPOUT, timing set a 10 btdc.
The data link is one plug it does not have the single wire anywhere in this harness, using jumper wire, no codes found.

Suggestions beside a match to the gas tanks?
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Tuesday, March 2nd, 2021 AT 12:56 PM

9 Replies

Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 108,179 POSTS
Hi,

You've been through a lot. I attached the only vacuum schematic I could find for a 1989. Take a look through it and see if it helps.

Here is a link to help with finding a vacuum leak:

https://www.2carpros.com/articles/how-to-use-an-engine-vacuum-gauge

The vacuum is slightly lower than I would expect. Usually, it is between 18 and 22hg. Does it stay steady when the engine is loping?

Have you checked for codes that may help? Here is a link that shows how to retrieve codes on a Ford OBD1 system. I realize you indicated you can't find where to check it. According to my manual, (F-Series), the Self Test and STI connectors and are located in the area of the EEC system charcoal canister.

https://www.2carpros.com/articles/ford-lincoln-mercury-obd1-1995-and-earlier-diagnostic-trouble-code-definition-and-retrieval

Also, the HVAC mode is controlled via engine vacuum, so check that as well. If you look at pic 2, it shows the vacuum schematic for the system.

____________________________

I hope this helps. Let me know what you find or if you have other questions.

Take care and God Bless,

Joe
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Wednesday, March 3rd, 2021 AT 6:56 PM
Tiny
DJKENNY619
  • MEMBER
  • 5 POSTS
A detail I left out, when the 5.0 was rebuilt, a 351W cam was installed.
The mechanical vacuum gauge pointer hovers at 17, I agree this is on the low side.
Leaving the MAP sensor vacuum hose connect all other vacuum hoses where disconnected and caps placed over the ports. The PCV hose was left alone.
There is a vacuum hose routing decal on the truck, I will compare with the one sent (thank you).
The EEC on the truck is located on the drivers side, there is no STI plug, kid you not, does not exist.
I traced pin #48 (white wire with red stripe) on the ECM to the test plug, looking at the plug its the bottom left wire.
True HVAC vacuum hose(s) or vacuum door can cause run ability issues, I ruled this out since the engine vacuum ports are capped off.
The vacuum gauge will fluctuate as the engine idle surges.
This is just as bad if not worse than a GM OBD2 code P0300 (random misfire).

Thanks Joe for your reply and diagrams. I will check/recheck tomorrow.
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Wednesday, March 3rd, 2021 AT 7:48 PM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 108,179 POSTS
Hi,

First, you're welcome for the pic. I have a question. If you plug the PCV vacuum, what does the vacuum go to?

The funny thing is it happens when warm and not cold. Does the engine coolant temperature sensor indicate a normal operating temperature?

Joe
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Friday, March 5th, 2021 AT 7:57 PM
Tiny
DJKENNY619
  • MEMBER
  • 5 POSTS
Cold operation is what I would consider normal. A fast idle at start, as it warms the idle reduces. At "normal" temperature the idle will start to *lope, hunt, fluctuate (what is the correct word for this situation)? Until it finally stalls.
I have not removed the hose to the PCV, I will do that now and try it.
I'm not looking at the gauge, I am feeling the temperature at the top radiator hose.
I don't have much trust in factory gauges.
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Tuesday, March 9th, 2021 AT 8:44 AM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 108,179 POSTS
Let me know about the PCV. As far s the issue, it may have to do with changing the cam and the computer seeing different signals.

Let me know. Also, if you have a vacuum gauge, let me know what vacuum is at idle.

Joe
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Tuesday, March 9th, 2021 AT 5:08 PM
Tiny
DJKENNY619
  • MEMBER
  • 5 POSTS
It's now with a retired Ford mechanic who worked on Ford vehicles since the OBD1 beginning.
He is 99% sure it is not the 351W cam in that 302, that is all I know at this writing.
Vacuum at idle, engine at operating temperature, before the lopping was 17hg, ideal range is 18-22.

Thank you,

Kenny

I will post details of the repair when, if available.
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Wednesday, March 10th, 2021 AT 7:34 AM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 108,179 POSTS
Kenny,

Thanks for getting back with me. I look forward to hearing what is found. Chances are it will help others as well.

Take care,
Joe
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Wednesday, March 10th, 2021 AT 4:51 PM
Tiny
DJKENNY619
  • MEMBER
  • 5 POSTS
Yes, I consider it unfair to come to a forum such as this one, seeking solutions and not sharing the results, where ever it's found.
As of this reply, the Ford mechanic is scratching his. With no answer in site.
. To be continued.
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Wednesday, March 10th, 2021 AT 5:34 PM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 108,179 POSTS
Hi,

We appreciate it. I am really interested in knowing what is found.

I will wait for the continuation. LOL

Take good care of yourself,
Joe
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Friday, March 12th, 2021 AT 11:03 PM

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