Engine Performance: Sudden drop in power during acceleration. Idles/starts well.

[rhinob]

A friend has asked me to check out his '85 M1009, 26,000 miles, all new filters and oil, transmission serviced and fluid / level good. During acceleration through 2nd gear, the engine output drops to about half as the limited acceleration continues. First gear, no problem - good power. Beginning of 2nd gear, no problem - good power. Half-way through 2nd RPM run-up, the power drops suddenly and this remains through out the run up to speed in the remaining gear to top gear.

Help?

 

[rsh4364]

Mine did something like this.Discovered the rubber fuel line feeding mech. lift pump had a tight bend,and as it aged it started to close itself off.I have since gone electric and much better.

 

[Keith_J]

Also check fuel sock in the tank, water in fuel causes bacterial sludge which then colonizes on it, obstructing flow.

 

[rhinob]

Thanks for the input.

When the power is lost, it's at the same RPM, in the same gear, and feels like a mechanical "clunk" of loss. It's abrupt and consistent. Does it make sense that it could be biological colony or a bend in a hose / hose deterioration? It feels like a solenoid is flipped on and it's purpose is to limit the throttle or something. Very consistent. Almost like an obstruction in the exhaust….full throttle, bogs down.

Originally I thought maybe the fuel pump was bad as when my friend has his truck serviced and returned, the oil level was about 5 quarts high. He corrected that and noticed these symptoms. I originally thought the diaphragm was torn and fuel was getting in to the engine oil. No such luck….the oil level has been normal for a while now and doesn't smell like diesel at all.

Racking brain...

 

[the skull]

Don't forget to check the return fitting in the top of the pump.
Mine was clogged, I was down to 35 mph and had a really
big blue/white cloud following me everywhere. Even idling
in the barnyard. Save some time, if its plugged, break the ball
and take the spring out. Mine runs a little strange but she's
at 60 in no time.

 

[Chaski]

Governor flex ring failure in the injection pump?
Injection pump check valve sticking or clogged?
Return fuel line or hose pinched, swollen or clogged?


Maybe pull the top off the pump and have a look...

If the flex ring had come apart bad things can happen, including plastic chunks blocking the check valve and causing the case pressure to rise high enough to limit the travel of the high pressure pistons.... Basically making it fall flat on its face. Same thing can happen if the return line is clogged or restricted.

If you pull the pump top be sure to install it wit the throttle shaft at 0 throttle and install the top parallel to the top of the pump, but diagonally from the front of the engine lowering it as you move the cover towards the rear of the engine. The reason why is to make sure you don't put the shutoff solenoid arm on the wrong side of the metering valve. If you do that you will have a truck running at wide open throttle with no governor until your engine becomes scrap.
.

 

[rhinob]

Superb advice, guys, thanks. I'll have a much closer look today!

 

[rhinob]

Pulled the air filter assembly off and removed the vacuum lines to the IP in an effort to clear the top of the IP for removal. That's when the vacuum lines crumbled in my hands.

Replaced vacuum lines and test drove. It runs beautifully and pulls hills as it should. Thanks for all the brain cells in an effort to think this through! It's nice when you trip over the problem so soon in the game!

 

[Warthog]

So it was the vacuum lines for the transmission and not the injector pump. Would not have guessed that by your description.

Good catch.

 

[rhinob]

Correct. Technically it was the vacuum supply hose from the pump to the IP. It was all a crumbly mess.