Ford Ranger 3.0 Vulcan idle knock

[eagle23]

I have a 1991 Ford Ranger 3.0 V6 Vulcan pushrod engine. It has original 75k (I bought it from the original owner).

It has an intermittent idle-speed knock up to maybe 1500 rpm. It can happen cold, warm, hot. It usually happens at idle without accelerating and will start knocking sitting at a light idling and the engine runs rough at the same time. But it isn't consistent.

I have replaced the throttle position switch, idle air control (since it's most likely to happen at idle) and the MAF sensor. All helped but did not cure. It does not throw any codes, no CEL. Timing was correct at 10 deg BTDC using the proper technique (with a certain circuit disconnected)

As it has no vacuum advance (it's all electronic) I am wondering if it might be the ignition module or maybe the ECU itself. The cap/rotor look good and plugs have
I have the impression this is not uncommon but I can't seem to find the cause and am not quite ready to go to the local dealer for diagnosis though I may have to.

Thanks!

 

[punisher]

Low speed mixture distribution problem in combo with the cam specs. Similar thing happened to the 4.6 modular so FoMoCo had another EDIS module designed to act like an MSD at low engine speeds.

I don't think you have a real/fixable problem. They did sometimes idle rough, and Ford never had a fix for intermittent rough idles on the 3.0/4.0 truck applications. It was considered "normal operation".

As far as the knock, I can't speculate. Best guess is slightly excessive piston skirt clearance.

 

[eagle23]

Thanks.

I also tried the "Just Ask" website and corresponded with a Ford senior tech who suggested replacing the module and pickup, with the ECU being a distant possibility. The module is where I was headed and since it and the pickup aren't terrible for cost, I'll give it a shot.

You may well be right about the "normal operation" reply from Fomoco, sad to say. The knock sounds like pre-ignition, like the timing suddenly advanced too far. Anyway, thanks for your reply.

 

[mechtech2]

It depends on what the 'knock' is.
Is it an internal engine part knocking, or spark knock?

 

[eagle23]

It sounds like a bad spark knock, as though the timing advanced a whole bunch. First time I heard it, I thought the exhaust system was banging on the chassis somewhere, but that wasn't the case. I have used MMO for 1k before the oil change and for the past few tanks and it's gotten a bit better.

The idea that the module may be malfunctioning and bumping the timing electronically sounds more and more like what's going on. The fact that it's so inconsistent makes it sound like an electronic glitch.

I didn't have the same problem exactly on a SB 350 a few years ago in a late-70's Chevy pickup but in changing the ignition module it cured it. But it to was inconsistent: sometimes rough idle, sometimes misfire and bogging on hard acceleration.

I am very open to other's ideas. They are much appreciated.

 

[DC84]

First off you may want to rethink using platinum plugs for your application. The older 3.0's were prone to spark knock. Many people including myself have had better luck running cheap copper plugs that are one ranger cooler than stock. Check out 3.0 forum at http://www.rangerpowersports.com

 

[eagle23]

DC84, those are terrific websites! Thank you!

I'll change the plugs to the cooler ones, Seafoam the upper cylinder area, and may change the thermostat to a 180. The old one was running way too cool when I got the truck and, of course, I did a bunch of things at once. I hadn't known the 3.0 was sometimes prone to those problems, yet there are a lot of potentially easy fixes.

 

[Buffman]

I used a lot of Chevron with techron fuel system cleaner, seafoam and MMO on my 91 Ranger as it had 240K+ on it. IIRC I pretty much ran 87 or 89 all the time, and I never had any spark knock. I was suprised at how nice and peppy it was considering how much power the vulcan didn't have. I'm sure the 3.45s out back and the 3.72 1st gear ratio helped it

I did run copper plugs in it. eagle23 is it a manual trans car? If so and you need a spare ECM to compare, I have a reman cardone I had in mine that I still have after I parted it out. My original ECU was fried by the water that leaked in on the A-pillar. After it wouldn't start I found the culprit when I sprayed the corrodded ECU with brake cleaner and it fried to life. I paid a ton for the cardone ECU, and protected it with a nice thick plastic bag and duct tape. If you want it, I can let it go for dirt cheap compared to new

 

[eagle23]

Buffman, it's an auto, not a manual. I'm not sure the ECU's would interchange but if it will I may well be interested.