Any ideas?

[79GLIDE]

The '90 has developed some strageness. Hopefully there will be some consensus on what it may be. For no apparent reason, the engine seems to surge 200 RPM's while at even speed, once it has shifted into overdrive. I notice it once I'm running more than 45mph. When it started, the truck did not have any kind of load on it. Say I'm running 65mph down the interstate. The motor seems to surge, showing a 200rpm increase on the tach, and within a few seconds it drops the same. Running on the flat, it will cycle frequently like this. Acceleration is smooth in spite of this condition, and the truck seems to pull away strong, leading me to think this is not a transmission problem. Otherwise, it idles good and runs smooth.

Friday, I was hauling 3200# of rock, and it did the same thing, and it seemed a little flat on power on the highway. I was wondering if this could be a problem with the governer in the injection pump, or if it's more likely an E4OD transmission control problem.

This evening I did notice the voltmeter jumping occasionally, as if the glow plug controller was cycling, but the truck was at operating temperature at the time, and I didn't have time to investigate any further.

BTW, if anyone is interested in pictures of Missouri River flooding near Omaha, let me know. I took a bunch.

 

[TWeatherford]

My first guess would be an air leak in the fuel system. If air is seeping in it will starve the pump a little, and it'll advance timing and throttle automatically to compensate. A line that holds fuel in just fine and doesn't drip may still leak air in if its under a slight vacuum, could even be a tiny rusty spot on the hard line.

Before you replace all your fuel lines, you could put in a sight glass to try to see the air, though it might be hard to see. I haven't been able to track down a good sight glass, so I use a Napa 3003 inline fuel filter, which is clear plastic, and costs $4. You could also hook your injection pump supply and return directly into a jug of very clean diesel, eliminating almost all of the fuel system for a test, though that would be hard to do under load.

 

[Dieselcrawler]

Is it possible for air intrusion? Air in your fuel lines will cause engine to surge.



Haha, posted at same time.

 

[79GLIDE]

I did replace the injector lines, including caps and line from filter to pump, last fall, and it had been running well until lately. I might try putting a clear filter on between the lift pump and the primary filter.