Yes, a 12.5 AFr will cause bad mileage- it's stupid rich. Something like 15% too rich.

If the iac is gummed, that may slow its response, but on the other hand, you were showing 10psi (10 inches manifold vacuum) at 2000 rpm and that is what doesn't add up. I'm showing 2psi (26inches) under vacuum (throttle closed coasting) and 6psi (18 inches) driving (throttle blades open).

In an ideal situation, and I can test this on my truck, with throttle blades wide open and engine at minimum RPM (before it zings), should register barometric pressure, 15 psi (30 inHg) since there is minimum vacuum drawn from the engine, and an endless supply of air. Then with engine at full song and the throttle abruptly closing, manifold pressure should do what my picture showed, pretty much slam to nearly zero if not negative, as there is a massive vacuum pulled against the closed throttle blades.

 

Here is why I say it doesn't make sense. You are roughly 15% rich, or I think I calculated like 3 pints rich or something. But you are showing a rather low manifold vacuum/high manifold pressure.

What this tells me, low vacuum = thing is not sucking as much = it's getting unmetered air from elsewhere OR whatever causes the sucking, is not sucking.

If it were the former, it would be a vacuum leak, and the motor would be lean.

If it were the latter, the engine simply is not sucking the air that it should be. As a result, insufficient air and same fuel = rich, which is what you have.

You don't want to hear my opinion, but my opinion is that I think you have a broken piston ring or rocker arm. Let me explain why.

If you had a broken piston ring, oil would make it into the cylinder on the downstroke, adding an unmetered fuel. At the same time, the oil would be coming up with air from the oil pan, meaning that less air would have to be pulled through the intake valve.

On the other hand, if you had a broken rocker arm, less air would be either exiting or entering the cylinder,measuring a rich condition.

My suggestion: key a compression tester gauge and also a leak down tester if you can.

 

Hi all,
just wanted to give an update as to our progress in our issues,

Since my last post, we have done the following:
-removed Air intake and checked/ resealed it to ensure their was no air leak, while doing so we checked the rocker arms and none were broken. As it was not burning any oil, we assumed the piston rings were fine.

After doing so we thought the problem was gone as we took it for a test drive and it did not surface, however a few drives later and the issue resurfaced.

-we unplugged the AC compressor, (heard on a forum that a mechanic had said this could cause RPM fluctuations, which we assumed to be due to electrical reasons?)

No difference.

-Changed all 8 fuel injectors.

Problem almost got worse with hard to start (like it wasn't getting fuel, causing extended cranking, however with a fuel pressure test it was getting sufficient fuel pressure and bleed back was less than 5 psi in 5 min.) after resetting the PCM etc by disconnecting the battery after changing the fuel injectors.

With all of this done, it had made no progress, and we pretty well gave up.

However we again began researching online, trying many different searches etc....... including what the symptoms of a bad PCM were, however they did not really match our issues, as well as a bad fuel regulator/ filter, again not matching our problem.



until we came upon a rarely spoken about issue,
found first here

posted by Kev2 about 3/4 the way down the page
https://dodgeforum.com/forum/1st-gen...injectors.html

to download the TSB uploaded by the kev2
TSB Durange, RAM, Dakota

A TSB actually affecting
-- 1998 - 1999 Dodge RAM
-- 1998 - 2000 Dodge Durango
-- and 1998 - 1999 Dodge Dakota.


The issue reported?
intermittent CEL reporting injector wiring faults from injectors 1-8, (P0201 thru to P0208

The fix? A PCM flash and installation of new software.

we thought, what the heck it couldn't hurt, so we took it in. The dodge tech did the flash, and until now neither RPM fluctuated nor the CEL came on, but now the RPM began fluctuating again, and the CEL is also with the same codes.


After a more thorough look at what the live data was doing while the truck was fluctuating, we noticed that the readings for the TPS position fluctuated each time with it even though the peddle was held steady. We decided to try changing the TPS (throttle position sensor) as well as the crankshaft position sensor and cam shaft position sensor (after reading they can cause RPM or similar fluctuations), we went to a junk yard and grabbed each sensor off a dodge the same year etc as ours, as well as a PCM in case that was the issue. First we tried a quick switch of the PCM, no difference, so we switched back. Then we tried changing the Cam shaft position sensor, no difference and maybe even worse (the one we got from the junk yard was in pretty bad shape), so then we changed the TPS, and no more RPM fluctuation issues, as well as no CEL. although a slight fluctuation at Idle, we changed the crankshaft position sensor (for a new one) and the Camshaft position sensor (for our old "good" one). Now it is running like new and the RPM needle is solid.



thanks

Donnyh13