Hey all, lurker here who could use some outside input.


I've got a manual 94 4.0 4x4 Ranger with 230k that has recently started idling poorly until thoroughly warmed up. From a cold start, the engine will idle high like normal, then begins slow down as it heats up, bogs down slightly and surges repeatedly until it settles at about normal and runs pretty rough until I presume the O2 sensors are hot and the engine can come out of a open loop. If I start it up warm, it idles just fine. No codes on my OBDI. The engine will cut out if you hit the brakes hard below a quarter tank of gas.


The plunger on the IAC was tore up so I replaced it. Also cleaned the MAF sensor, replaced the ECT sensor, and replaced a broken wire going to the ECT sender. I gapped some brand new motorcraft plugs to 0.060, no joy. It does have a Screamin' Demon coil pack, and I've got a motorcraft coil pack, that I threw in with no notable difference.


I think it's a vacuum leak: when its warm and I unplug the new IAC, the engine just keep on idling with no change. I cant find any leaking hoses with a smoke tester, so I went ahead and replaced the upper and lower intake gaskets as it looked like there was some blow by at the front end where it uses RTV instead of a rubber gasket.


This morning after the gasket change, there's not much difference. Still seems to try to slow down appropriately, but struggles to do so and surges to keep running until the O2 sensors can kick in. Is there something obvious I'm missing, or do I need to keep on with the search for the vacuum leak.


Any help is greatly appreciated.

**EDIT** Well I went out with some propane and couldn't find a leak that way either, pulled my IAC cable and now the idle bogs down. So I guess that rules out a leak, or I just need to let it warm up some more.

 

Welcome to the forum

Replace PCV Valve as well, if dirty it can act as an intermittent vacuum leak until oil residue inside it warms up and valve inside can move smoothly

Could be computer issue as well, kind of a pain to pull it out but worth while after 26 years
This does two things, first when you unplug its connector and plug it back in it cleans the contacts, if ECT sensor voltage is varying(at the computer) because of poor contacts that would explain the RPM issue, once above 140deg varying voltage wouldn't matter, warm idle is set
Second thing is you can inspect the computers circuit board
There are 3 capacitors that can leak after 20 years or so, this can cause odd issues, there is no "thats the reason" for a bad capacitor, just odd unable to explain issues, lol
There can also be corrosion on the board, easy to clean off

In 1994 computer is in engine bay below brake booster over near inner fender and in a cavity in the firewall
You do need to move cruise control actuator, and unbolt the large round connector in firewall, just 1 bolt in the center
Computer connector also has 1 bolt in the center
Then there are 2 nuts above and below computers plastic holder, lower one is hard to get to, need a wobble on a socket, but once loose fingers can unscrew it usually

I just had to replace the 3 capacitors in my 1994, 2 were bad but replaced all 3, under $5
I had odd symptoms leading up to this but then had major symptoms, lol, including fuel pump shutting off

 

Interesting. I thought PCM function would be an all or nothing deal considering it’s age.

I did have PCM issues last year secondary to moisture under the connector leading to the truck going into a limp mode intermittently.

I also had some welding done on the frame a couple months ago so that might have antagonized any issues. Will pull it tonight and take a look.

Will report back with findings.

 

Here is a picture of my 1994 PCM: https://www.therangerstation.com/for...pcm-jpg.44443/

The one cap was bad and I could see it, I bent it over
But the one in upper right was also bad but harder to see until I was desoldering and it just pulled off, lol

Replaced the 3 and all has been well but only been 3 or 4 months

 

Pulled the board, replaced two capacitors that had severely corroded connections. So corroded that one broke off just touching it. First time I ran it after the swap it seemed to idle much better.

However now I’ve got codes 998 and 157, system in failure mode and MAF sensor low voltage, respectively.

Hoping I just pushed the pin out of the connector a touch, and that it’s not a dead PCM. Will test it the pin out and follow up.

Any advice going forward?

 

Pretty hard to kill these older computers

157 Mass Air Flow signal is/was low or grounded - MAF
998 Did not pass Key On Engine Off test yet (Get 111 in KOEO first)

If MAF was unplugged when computer was first powered up you will get 157
Check MAF connector

998 will go away or you can clear it as well to see if you can get 111 with key on/engine off

 

KOEO gets me 111, then when I turn it over I get 998 and 157. MAF connector appears good to go.

 

That is odd

Not sure what to say on that beside have another look at computer to make sure new solder didn't cause a short or something else needs attention
You didn't have these codes before the new caps?

But you said it is running good or just OK?

 

I realized I hadn't let the engine warm up prior to performing the engine running test. The codes and check engine light cleared after holding 2500rpm for two minutes like the book instructs prior to an engine running test. The only code I'm now getting is 30, which I cannot find anywhere in the book that came with my reader, or google. 111 on KOEO. You wouldn't happen to know what that is would you?

The cold idle is way better. Still bounces slightly, but appears to bounce very narrowly around a decreasing average RPM. I.e. the average RPM appears to be exactly what it should be, and I think the bouncing is the IAC duty cycle being relearned. Will update if it never normalizes.

Appreciate the help so far!

 

And you won't find it because OBD1, 2 and 3 digit codes, never used any "0"s, no way to display a "0" with a voltage pulse
OBD2 used "0"s

Computer will pulse a 3 after startup if its a V6 engine, or a 2 if its a 4cyl

Not sure why your code reader showed that

 

Awesome, I figure it’s an artifact of the code reader then.

So far seems my problems been resolved. I really appreciate your help!