720 SD22 Surging Idle

[SLO720]

Hello everyone, It's been a couple years since I last posted. A lot has happened and I have acquired an SD25 transplant engine. Anyways, I sold one of my SD22 engines to a guy out of Missouri the motor ran great and he put around 1200 miles on it before having to park it because his rear axle locked up. He parked it for about a month and it now has a really odd surging idle. I had him change the fuel filter, add a clear inline filter (wix 33006 at the tank to check for rust or contaminates), he also replaced all vacuum lines and it is still having the same problem. I'm trying to help him fix his issue but everything he has tried hasn't helped the situation. His truck had a sd22 in it originally but his motor was ran low on oil and I sold him a replacement motor. The truck will not move easily in 1st or reverse and at higher rpm it does not have a idle/surging problem. If he slowly releases the throttle it will also idle somewhat normally which leaves me really confused also. If he releases the peddle quickly it will return to surging. Maybe a problem at the throttle body butterfly?

 

Any help would be greatly appreciated! Thank y'all in advance!

[wayno]

I would suggest it likely is one of 2 things, either the hose going from the throttle body to the back of the injection pump(the front hose is a vent hose) has a crack/hole in it, or the diaphragm has a crack/hole in it, the idle would tend to surge because of being rich and the exhaust would likely be black.

Keep in mind that the venturi at an idle creates a lot of vacuum because the engine is running, if the vacuum hose has a crack/hole in it it looses this vacuum and the spring the vacuum is pulling against using the diaphragm pushes the injection pump to a richer position, since it cannot get enough air it surges, same thing happens if the diaphragm has a crack/hole in it.

It's easy to check the hose because it is on the outside if the engine, just move them around while it is running, if the idle changes it likely a bad hose, if nothing changes then it likely the diaphragm in the injection pump, I don't know any way to test that, there likely is a way but I don't know it.

If it is a bad diaphragm you could use one from one of your extra engines/injection pumps your not using.

Edited September 10, 2019 by wayno

[SLO720]

Yeah, I mentioned something about that to him too. I was just hoping that diaphragm wouldn't be the culprit. He told me that he would check the one on his old injection pump and investigate possibly using that diaphragm if he can get it out in one piece just to try it. I'll tell him to look over his vacuum hoses but he just replaced them too. I like these motors because they rather simple. Is there any for sure place to order one online? Or just pass him the zexel part numbers and try a forklift/diesel rebuilder? He keeps having problems with this truck and I'm doing my best to help him work them out. I'm still learning myself and the input is greatly appreciated, thank you! I think I read one of your posts on nissandiesels forum about cleaning the lift pump primer and had him clean that too just in case. The various nissan forums have been a great wealth of knowledge to me working on these trucks they are great places for sure 🙂 

.

[wayno]

Did things go south right after he changed out the hoses?

 

 

Edited September 10, 2019 by wayno

[SLO720]

No sir, it happened before. 

[wayno]

I have never had a diaphragm go bad so I don't really know all the symptoms, but surging sounds like something is wrong between the throttle body and the diaphragm, make sure there is nothing in/blocking that venturi orifice in the throttle body also, it needs vacuum to pull the rack back to the idle position.

There is a screen in the lower banjo fitting on the lift pump/primer pump, but if your running a pre-filter that screen should not be an issue, and it would just die if it got plugged up anyway.

[SLO720]

Thank you for the diaphragm emphasis as that was his problem. The diaphragm was torn, he swapped the torn one with the one out of his old motor it only had 109k miles on it and it looked really nice. He started it back up and it runs super smooth! What oil do you suggest to lubricate the diaphragm? Thank you again sir!

[SLO720]

[wayno]

I believe they say use "Cod Liver Oil" on the vent side of the diaphragm.

Be careful with that screen in the banjo fitting, I have not been able to find a replacement.

[wayno]

Oh, I am happy he figured it out, it's not that hard to get out if you have done one before, but it is a hassle to remove and replace it while it is in the truck, there is this small clip one can loose.

 

[SLO720]

Is there anywhere to buy diaphragm new? For later down the road, may even try to pick up one for my sd25 if they're different. Thank you again Wayno!

[wayno]

22 minutes ago, SLO720 said:

Is there anywhere to buy diaphragm new? For later down the road, may even try to pick up one for my sd25 if they're different. Thank you again Wayno!

 

I bought a couple of them from a guy selling them several years ago, as I recall he said they came from a diesel powered Mercedes, I have not seen or heard of any for sale since, maybe a forklift place has them.