My 1979 300SD has a "hesitation" when going 60 mph. It runs fine on city streets and has this problem only during highway driving. It is intermittent. The car has documented less than 150,000 miles and is otherwise perfect car. My mechanic changed the fuel filters without improvement. When he drives it is fine. The problem is real however and I'm afraid to go on any trips with the car until i resolve this problem.

Thanks in advance for any advice.


Steve Weis

#1. Biobomb the fuel tank.
#2. Remove the fuel cap and take it for a drive.

I suspect fuel issues.
#A. A plugged tank strainer.
#B. Vacuum forming in fuel tank.

Have a great day and let me know what has happened.

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Whunter, I have seen this many times, but I don't think I have ever seen a good explanation of why it matters.

My 84 SD has recently been giving me a vacuum 'hiss' when opening the tank. Not a big one, but just enough that I now notice it.....what's it trying to tell me?

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Fuzzball,

Your car is telling you that the tank vent is plugged.

When you get enough vacuum in the fuel tank, the fuel pump can no longer supply enough fuel to the engine to develope full power.

P E H

random power loss at highway speed is a good indication of this issue.

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__________________
ASE Master Mechanic
asemastermechanic@juno.com

Prototype R&D/testing:
Thermal & Aerodynamic System Engineering (TASE) Senior vehicle instrumentation technician.
Noise Vibration and Harshness (NVH).
Dynamometer.
Heat exchanger durability.
HV-A/C Climate Control.
Vehicle build.
Fleet Durability
Technical Quality Auditor.
Automotive Technical Writer

1985 300SD
1983 300D
1984 190D
2003 Volvo V70
2002 Honda Civic

https://www.boldegoist.com/

That's about the speed where 4th gear starts to bottom out in my 240D. Hell, I'm practically always dropping into 3rd at/near 55mph. And 4th gear never sees anything at/below 50mph.

Maybe your 116 automatic tranny is beginning to search for a lower gear at 60mph. Meanwhile the torque converter starts slipping and sliding to make adjustments that compensate for less torque at lower rpm's which is what torque converters are designed to do. I believe this is where/how your car's perceived "hesitation" might be occuring.

I dont think there's anything wrong with your car, maybe you are just being too picky and judging it too severely.

Roy is correct with the diagnosis of this problem. Either the tank vent is plugged and vacuum in the tank is preventing sufficient fuel pressure at highway speeds, or, the tank strainer is partially blocked and is effectively doing the same thing.

As he recommended, drive the car at highway speeds and see if you can duplicate the problem, or whether it has now disappeared.

I have a similar problem with the SD. I could not start it because of vacuum in the fuel tank.

We changed the 2 filters in the front. No change.
I drove it with the fuel cap loose. No change.
My mechanic drained the gas and took out the screen in the tank and said it looked fine and put it back in.

The problem is gone.

Thanks,

steve

I had this exact problem, and after chasing down the tank vent line, blocked tank filter, blocked line filters, etc. I found it was an intermittent air leak in the old lines in the engine compartment. Reseat the hand pump a couple of times and replace the 18 inches worth of various fuel line the the engine compartment. When you really get going the pumps suck hard enough to draw miniscule amounts of air, and it just won't make the power... My .02