Electrical Issue - Decel Fuel Cut (Long)

[Marc311]

This has been a long running issue and I have posted about it previously with no luck. Maybe additional info will help.

I've traced my afterfire (backfire) issue to the decel fuel cut system. I have the opposite problem others have - my fuel cutoff solenoid never cuts off the fuel.

For those who don't know - The Decel fuel cut system works by the emissions computer (ECU) telling the fuel cut solenoid to stop the fuel (by opening the ground) in the idle circuit when the truck is decelerating. The ECU does this based upon inputs from the Vacuum Switch and the Coil Igniter. The Vac Switch tells the computer when you are off the gas and the Igniter tells it the RPM as it only does this in a specific RPM band.

Here's what I know:
The Vacuum Switch is good (tested and tried others)
The wiring from the VS to the ECU is good
The Fuel Cut Solenoid is good
The wiring for the Fuel Cut Solenoid is good
The ECU never opens the ground to turn off the Fuel Cut Solenoid - THIS IS THE PROBLEM

Additional Info
The ECU should be good (no way to test, but tried 3 different ECUs)
The wiring from the ECU to the igniter has continuity. It gets voltage that slightly decreases with RPM - but I am not sure what it should be seeing. My guess is the voltage is actually pulsing 0 V to 12V and the decrease in voltage is actually just my multimeter not reacting quickly enough to the 0V which it is seeing more frequently as the engine revs.

So, if my ECU is good, the Vacuum Switch input is good, the solenoid is good, the problem may be in the igniter circuit. But the wiring appears fine, the Tach works, and I haven't the slightest how to test it.

Am I missing something? Could the culprit be somehwere else?

 

[2mbb]

You tested the vacuum switch. But did you test that the vacuum applied to the switch is correct? If the switch is connected to the wrong vacuum source, or if the vacuum hose or piping has a leak or if the throttle plate doesn't close enough to expose the vacuum port to manifold vacuum, the switch won't get the proper vacuum. Have you put a vacuum gauge on the line going to the switch to see that you get the required vacuum?

 

[Marc311]

I like your thinking 2mbb.

I have checked the vacuum and wondered the same thing. It appears to be hooked up to the right port and the vacuum goes from 0 to 15+. All new vacuum lines as well. I also wondered if the issue may be related to some sort of delay or slow reaction to the vacuum changes. It certainly won't hurt to check again.

I've looked at that same page of the FSM a thousand times. But one thing just struck me - In the last scenario (above 1800rpm, high vacuum) it says the slow circuit in the carb is closed. How does the computer know that? Doesn't the slow circuit valve get actuated mechanically by a lever attached to the throttle plate?

 

[Marc311]

By the way - here are some specific readings I took back in August when I was first dealing with the problem (as copied from an earlier post on the problem):

________________________________
Here are my vacuum readings taken at the vacuum switch using a mightyvac as the guage:

idle=17"
If reved quickly vacuum goes to 0

If slowly reved, vacuum increases to about 20" just shy of 1000 rpm, then drops to about 15". As I go up to 1200rpm, vacum drops to 11", 1500 rpm = 5".

As I let off the gas, vacuum jumps up to 22-23" before going back to 17" at idle.
___________________________________________________

These measurements may have changed a bit sine I have since adjusted the valves, but I imagine they are similar to what I would measure today.

 

[2mbb]

I like your thinking 2mbb.

I've looked at that same page of the FSM a thousand times. But one thing just struck me - In the last scenario (above 1800rpm, high vacuum) it says the slow circuit in the carb is closed. How does the computer know that? Doesn't the slow circuit valve get actuated mechanically by a lever attached to the throttle plate?

I think the last column in the table is referring to the result of the conditions applied. I don't think it is referring to the position of the slow cut valve.

 

[Marc311]

How about anybody else out there? Any left field ideas?

The engine is otherwise running well and passed smog with flying colors. But something is clearly sending a strange signal to keep the fuel cut from closing.

Heck, if anybody can think of some other way to rig the fuel cut to go off at the right time, I would take that as well!

 

[NocalFJ60]

So all 3 ECU's gave the same results? Check the ground on the ECU board. Maybe a bad connection on the ground pin? You could run another ground wire direct to the board and bypass the connector's ground.

 

[Dynosoar]

Is your speedo working? You do have a VSS (Speed Sensor) attached to back of the the gauge that sends a signal to the computer.

Dynosoar

 

[Marc311]

So all 3 ECU's gave the same results? Check the ground on the ECU board. Maybe a bad connection on the ground pin? You could run another ground wire direct to the board and bypass the connector's ground.

It's nice to see others are thinking the same way I did. I did check that and it is good. But, really, my problem is not that the computer has a bad ground, it's that it won't OPEN the ground for the fuel cut solenoid.

Is your speedo working? You do have a VSS (Speed Sensor) attached to back of the the gauge that sends a signal to the computer.

Dynosoar

My speedo is working and I do have a VSS. I don't believe vehicle speed is part of the fuel cut system. BUt I did check it - I think in relation to checking the CAT system when I was smoging it.

I'm going to check my vacuum lines (for the thousandth time). I was just looking at both the carb layout in the 2F FSM and the emissions FSM. Although I have good vacuum at the vacuum switch, the only thing I can think is that I am hooked to vacuum that changes at slightly different times so the ECU does not know when to activate the fuel cut.

EDIT: It is defintiely hooked up to the right vacuum source. Drat!

 

[Kavik]

Update?

Can someone post a pic of the vacuum switch?

Edit - found a pic

 

[Marc311]

Not much of an update.

Although the Decel cutoff circuit is not functioning properly, there are numerous people that have grounded theirs (which basically makes it act like mine) and they don't have the afterfire problem. Of course, they may also not be running a cat, but who knows.

So along those lines, I wanted to be sure all other things were working properly so I was atleast getting efficient combustion. Since afterfire is unburned fuel burning in the exhaust, I wanted to be sure I didn't have any other issues. My plugs are less than 1000 miles old, but I checked them and they all looked good and gaps were good. Wires are also new. Coil tests fine, but who knows if a hotter spark could help.

I had adjusted the valves (actually a couple times) around the time this problem started (but before I replaced the cat). The new cat is definitely what made the problem apparent, but that is probably because the old cat did not get hot enough to burn the fuel.

Anyway, I had read elsewhere that tight exhaust valves can lead to innefficient combustion. So this time, I adjusted them with the engine running. Sure enough, all my exhaust valves were too tight. So I readjusted them - erring on the loose side. That definitely lessened the afterfire problem. I never got the shotgun sounding backfires before, but I would occasionally get pretty loud pops. Well, those seem to be gone. That said, I still get the frequent, albeit much milder, popping on deceleration. Some of the sound I might be willing to attribute to the Magnaflow muffler, but it only starts once the exhaust heats up.

I just put a can of BG44K in the tank to hopefully help with any carb issues.

So, the condition has gotten a bit better, but the underlying problem is still present. I have even toyed with bypassing the ECM and trying to develop some sort of switch myself to control the fuel cuttoff.

It really has me baffled. I can't help but think it could be something really simple that I am just overlooking, but I've posted about this a couple times and I think if I missed something obvious somebody would have posted about it by now.

 

[Marc311]

I bypassed VSV2 which solved the afterfire problem. There is still some underlying issue, but at least I now have a temporary work around. The VSV and the FCS are related and the only common denominator seems to be the ECU (or some input for the ECU). I'm happy to be rid of the "pop pop" but would still like to solve the problem.

Good enough for now I guess.

 

[gaylon]

I am in Baltimore with two running 60's that do not exhibit the problems you are experiencing. If you want to come up on a weekend and compare how they are hooked up and running and see if we can figure out how yours is the odd one out, I am open to it. Just let me know. One of the trucks is for parts, so if you end up needing something you would be set.

G

 

[Marc311]

Thanks Gaylon. That's a generous offer. Everything is hooked up per the FSM (I've checked about a zillion times). Of course, who knows, maybe I just need to try a fourth ECM!

Thanks for the offer. If I continue to be stumped and the FSM doesn't help, I may have to take you up on the offer (and might have to check out that trailer!)

 

[gaylon]

Okay, just let me know. I am sure we can find a few to knock back while looking. Actually, I will be dismantling the second 60 soon, but even with one and one I am sure we can work on the problem.

G

 

[hank14]

Any update on this problem? I'm experiencing similar issues after rebuilding my carburetor.

 

[hank14]

Any update on this problem? I'm experiencing similar issues after rebuilding my carburetor.

I'm a doofuss!

I had the top 2 vacuum lines from the 4 PS carb ports reversed. The distributor main advance was getting the idle cut vacuum since reinstalling the carb. I even thought I double checked those things when I re-installed the carburetor.

Good news is, my 60 is running smoother than ever after the carb rebuild.

 

[hank14]

I'm still a doofuss.

And...I still have the afterfire problem. It is only when the engine is cold. I think it may be related to an exhaust leak, but I just rebuilt my intake and exhaust manifold and put new Remflex gaskets on.

Anyone know why I would only have afterfire on deceleration when the engine is cool or cold and it goes away when the engine is warm?

 

[hank14]

Update to my problem mentioned in this thread for future reference. I found out my EGR modulator valve was not working to FSM specs. Replaced it and the afterfire is gone.

More info in this thread:

https://forum.ih8mud.com/60-series-wagons/416057-fj60-rebuilt-carb-now-engine-backfireing.html