4.6 Stroker runs rich when hot

[NelsonATC]

I have a one and a half year old stroker that has an issue with running rich when hot.

First, some background:

4.6 L stroker, 99 block, 99 head (mild P&P), adapted FI from 94 Cherokee, with 2001+ intake and a 63mm TB and Cobra 24# injectors in a 1980 CJ7

Runs great for the first 1/2 hour of constant run time, but when it runs for longer periods (prolonged highway, long slow trail rides) it starts to run rich (AFR around 10, give or take). Bringing RPM's up will get it back to 14.8, but at idle or low steady RPM's, its still hangs around 10.

It is my belief that this is a heat soak issue. To date, I have switched to a dual electric fan, run hood louvers, and heat shielded the intake manifold, fuel rail, and injectors. This made some difference when I did it last year, but the issue is still there

I tend to see this more driving around town, as on the trail its stop and go because of the obstacles and such.

Any thoughts on how to deal with this?

[jsawduste]

Is the ECM getting a VSS signal ?

[Cheromaniac]

Check the O2 sensor signal when the engine's at hot idle to see if you're getting the proper voltage.

[SilverXJ]

I don't see how its a heat soak issue unless there is some kind of problem with the injectors. I'll recommend what Cheromaniac said, check the O2 sensors.

[NelsonATC]

O2 sensor was replaced with no change.

VSS is functioning properly

Any other thoughts?

[Cheromaniac]

You might wanna check the intake manifold vacuum and rule out the possibility of a vacuum leak.
Also check if the vacuum tube from intake manifold to MAP sensor is either cracked, broken, or disconnected.

[NelsonATC]

Reviving this topic, because I still haven't quite resolved the issue.

All suggestions previously posted have been checked and are functioning properly.

A little more to add to the equation:

I have noticed that during cooler days, the issue is not quite as frequent. However, yesterday, during a 90° plus day, the issue cropped up much faster.

If I go on a long highway drive on a cool night, everything stays normal, but if I took the same drive on the relatively hot afternoon, after about a half an hour of consistent driving, the air fuel ratio will drop off, and she will run Rich, to the point where I can smell it in the exhaust fumes.

[BADASYJ]

You did change your cap, rotor, plugs, n wires correct?
It sounds like its triggered by a reletively minimal temp change. ( night verses day) Which I think would rule out head and header distortion. Intake manifold distortion causing an intake leak also sounds far fetched as it should develop at night as well. It has to be electrical. I'm thinking maybe a heat soak issue within the Computer itself. Could try icing it to drop temp next time it acts up and see if it changes anything.

[NelsonATC]

Cap and wires done correctly.

Will investigate the ECM issue the next time I take it out for a longer ride. Will report back on findings.

[jsawduste]

Heat soak on the ECM is very remote.

I'd be looking at a poor coolant temp sensor and its wiring. Think open vs closed loop and what changes between the two modes.