Engine/Transmission (1994 - 1998) AFC could be the problem!

[KMott]

OK, so i think i found the problem! After having ML truck service replace the afc diaphragm and the wategate actuator, i wanted to make sure that the afc was actually working. So i unhooked the line that runs into the injection pump. I ran a longer line into the cab of the truck and hooked a pressure gauge.

On the injection pump (where the line would have gone) i put a line on and plugged off the end ( i tried this before without plugging off the injection pump, and it really lost power)

anyways the first thing i noticed when i drove it was it didnt run any different. It had the same amount of power in every way. Next i checked the gauge and there was no boost at all going through the line. So basically this means that my AFC diaphragm is not moving at all because there is no boost in the line. I just want to make sure i understand this right: The boost line carries tubo pressure all the time or just when the wategate opens? And when the Afc gets boost, it is forced forwards (against the spring) and opens up the fuel, and this happens gradually or all at once?

So right now my AFC diaphragm isnt moving at all, and therefore not letting it get full fuel.

I just dont understand how a reputable shop can randomly replace parts at my expense in an attempt to fix a problem they didnt even fix!

Thanks for any input on this, i cant wait to talk to them about this.

 

[Joe G.]

Wow! That is a complicated way to test the AFC. After reading it I don't really understand what you did or what you are trying to do. The easiest way to test the AFC is to take it off. If the AFC is not working and you take it off the truck will have a lot more power and smoke when you step on it. Boost pressure is supposed to push the AFC throttle stop forward. This will allow the governor follower to use the torque plate as a throttle stop. If the AFC is not working then the AFC throttle stop will not move out of the way of the governor follower. So if you remove the AFC there is nothing to stop the governor follower from contacting the torque plate, boost or no boost.



So to test it, remove it and cover up the hole with something to keep dirt out and oil in. You can leave it attached to the boost tube. Just hang it from something so it does not flop around. Go for a ride. If it goes better and smokes a lot then the AFC was the problem. If there is no change then the AFC was not the problem. Pretty simple.

 

[KMott]

All i did was disconnect the boost supply from the injecion pump. And it ran the same with the boost line off. Which means that the afc is not working? I guess i can try it your way. You are saying to just take the 4 bolts off so the rear housing comes off (so you can see the diaphragm) and cover the hole that boost would normally come through?

 

[Joe G.]

If you take the boost line off that is exactly the same thing as breaking the AFC. The AFC functions as a throttle stop. That it is stops the governor follower from allowing increased fueling until there is boost. NOT the four bolts off the rear. The four bolts that hold the entire AFC housing down to cover the torque plate. One of them is the "tamper" proof screw that has to be removed to change plates on the pump.

 

[Joe G.]

Look at the picture in this thread:

https://www.turbodieselregister.com/forums/showthread.php?t=158872

It shows the AFC thottle stop in contact with the governor follower. It also shows the torque plate. In the picture you can see that the AFC throttle stop is in the way of the governor follower so it can't move forward. If you lift the entire AFC off you can see that the governor follower is not prevented from moving to the torque plate.

 

[KMott]

That clears things up a lot for me. I'm going to try that tomorrow morning and see what happens.



I feel kinda dumb because what i tested earlier didn't mean anything at all. For some reason i thought the boost came from the wastegate over to the injection pump. (thats why i pulled the rubber line that comes from the wastegate off the pump and stuck a pressure gauge on it)

And i guess when i plugged off the fitting on the pump i was just sealing the boost in the pump, and preventing it from going to the wastegate. Hopefully I didnt hurt anything



Thanks again for all the help, I owe ya!

Kevin

 

[Joe G.]

The boost pressure is used to move the AFC throttle stop so when you removed the line you stopped it from doing that. Thus the AFC throttle stop would stay put which may be the problem in the first place. It did not hurt anything.



If removing the AFC does not fix the problem then we have to look somewhere else. If it does fix the problem then it's very easy to check out the AFC with it off. Set your air compressor to 20 PSI. Apply air to the boost fitting on the AFC. The throttle stop should move. If it does then you are not getting boost to the AFC. If it does not move you have some sort of problem in the AFC, probably a bad diaphrame. These things are simple to diagnose if you separate the problem into pieces.



As always, I advise that you find what is wrong before buying parts to fix it.

 

[Howard Durand]

The way I understand AFC operation is that the force that actuates the AFC comes from it's direct connection to the intake manifold through a metal line.

To modulate this pressure, there is a tee at the AFC that provides connection to the turbo through a rubber hose.

TST, and others have designed an adjustable elbow to be placed in the line between the AFC and the turbo so turbo effect can be adjusted.

As flow through the elbow is reduced, boost pressure to the AFC is increased. If you disconnected the hose that runs to the turbo and replaced it with a short hose with a plug in the end, you should be getting maximum AFC response.

If not, maybe the line from the manifold is defective. Or you may be losing boost elsewhere.

 

[KYOUNG]

Your steel boost pressure line that goes from turbo to AFC could have a crack in it, check it out.....

 

[KMott]

I got it all pulled apart and took it for a drive, and it ran the same. AFC is working fine! While i was in there i checked the governor lever and noticed it was hitting the fuel plate near the middle (maybe . 2" under the lip that stops it) . Thats at maybe 1/8 throttle where it hits the plate, and as increased the throttle a bit more maybe 3/8 throttle, the lever rode up and hit the stop. On the weekend im going to reset the govenor so it hits at the bottom of the plate instead of the middle:

http://www.piersdiesel.com/TechLeverAdj.htm



I can see how a different fuel plate could make a difference. There really is no curve to the stock one. It just hits a flat and rides up it for a quarter inch. It has nowhere to go after less than half throttle.



Not sure if it matters but my fuel plate is also bolted on a bit of an angle, and is slid most of the way toward the rear of the truck.

 

[Joe G.]

The stock fuel place should be about in the middle and it should be straight. Moving it forward increases fueling. Back toward the firewall reduces fueling.



The AFC appears to be working fine, but you don't know for sure unless you test it with some air. It's a good assumption that it is in fact working ok. If the fuel plate is moved far enough back the AFC may not have much to do anyway.



Most of the motion of the governor follower is in response to RPM. The higher the RPM the higher the follower contacts the fuel plate curve. That's the reason for the various shapes of the curve. The maximum rack travel is different. It is controlled by the curve depending on RPM.

 

[KMott]

I straightened out the fuel plate and slid it forwards a bit, and i also pushed the AFC housing forwards all the way as well. This didn't have any effect on how it ran, and it still kind of sputters when it takes off.

To measure boost, I put a t-fitting on the rubber line off of the injection pump that goes to the wastegate and measured about 10 lbs boost at 2000 rpm in o/d and it maxxed at about 12 psi when it revved out in each gear.

Not sure if thats where it should be or not, but my manual says the wategate open at 19. 3 psi, so i would think there should be more boost in the line.

Thanks again.

 

[Joe G.]

18 PSI is about what to expect with a stock engine. You may have a fuel supply problem. Check your fuel pressure while the truck is on the road. Minimum you should have in those conditions is low 20s PSI.



If you suspect a fuel supply problem read this:

http://dieselram.com/ubb/ultimatebb.php?ubb=get_topic;f=45;t=000007



You may also have a boost leak somewhere. Very carefully check the boots on the bottom of the intercooler. They can rub on things and leak. Make sure all intercooler clamps are tight. The service manual says the torque on the clamps is 72 inch/lbs.

 

[KMott]

I already printed off all of that information a couple weeks ago and went through and thoroughly tested the fuel supply. Pressure running down the road is 30-40 psi even when under full load. I even checked the vacuum before the lift pump and got 5hg, as the manual specified. I pulled the tank and cleaned the filter, cleaned the pre-filter, replaced every rubber line, replaced the overflow valve, fuel filter etc. I did do a fuel volume test, and it seemed kind of low compared to what the haynes manual said (sorry i didnt throw the haynes manual out yet... . lol) but since fuel pressure is good, i figured that was not an issue.



As far as the tubo, I checked and tightened all of the clamps and inspected all the boots last weekend. I thought one was worn through because it was rubbing on an electrical connection, but after i got it off i realized it was only about half way through. I also looked over the intercooler and couldnt see any cracks.



Im suprised though, that with such low turbo boost the thing still doesnt smoke black (especially after moving the fuel plate forwards today) Anyways tomorrow morning i will re-check the boots by the intercooler and see what i can find. Im kind of hoping its just the turbo needing rebuilt, because i have a lubrico warranty still, that says it covers turbochargers. I did notice that when i shut the thing off it sounds like the turbo is grinding a bit when it slows down. (since i have no muffler it easy to hear) Thanks again for the help.

 

[Howard Durand]

This has become a very interesting thread. Sorry your truck is the point of interest without having gained any real improvement. The 10 lb. boost you measured at the AFC fitting seems low. I had a situation where boost dropped off and found it at the first boot out of the turbo. The lower end of the boot is cut at an angle and it was situated under the clamp looking like it was OK but it was actually almost out from under the clamp. Just enough to leak.



One thing that I would like to have cleared up is the sequence of events regarding the boost line that runs from the intake manifold to the AFC and then on to the waste gate. I am assuming that boost from the manifold operates both the AFC and the waste gate. If the line to the waste gate is plugged or removed, the waste gate will not open but AFC will still operate normally.



Back to your problem. It appears you are getting adequate fuel from the source but it may not be getting to the pump or not getting through the pump. Have you checked for full travel of the fuel solenoid? If so, is it adjusted properly? How about throttle linkage?

Beyond that, I don't know how you determine if the pump is actually delivering the right amount of fuel to the injectors.

 

[KMott]

Howard, it took me a while to figure it out, but your right about the way the boost to the AFC works. The little metal line off the intake that runs over to the AFC supplies the boost. If you plug the wastegate line there is no way for the wastegate to operate, but the AFC will still work. If you pull the rubber line off the pump that runs to the wastegate you will have no AFC or boost pressure because it will all just leak out. In my case it wouldnt even matter if i plugged the wastegate because my turbo isnt even making enough boost to open it. BTW i was measuring boost from the line that runs to the wastegate.



As far as the fuel solenoid, believe it or not, already been there and done that. I put a brand new one on and fixed the problem of it barely opening. It now opens full travel. Also i had my cable replaced while getting the 4 recalls done to it about 2 months ago, and it is opening full.



Maybe it will be worth spending the money to get the pump tested. But that will have to wait for a while.

 

[KMott]

I just talked to the guy at ML who replaced the AFC diaphragm and wastegate actuator. He said they did an air to air test on the turbo system and the only leaks that were there were the two parts they replaced. He is saying that i need to have the 'rack adjusted' or something like that on the injection pump to increase the fuel pressure. He cant set it, but the diesel injection shop can. Basically since im not getting enough fuel, im not getting enough turbo pressure. If it was a turbo problem i would be getting black smoke, and im not. I know that the AFC has to open before full fuel, but it only takes a bit of boost to open it up (and im getting that bit of boost).



So, do you think i should take it down the the injection shop and have it looked at?