Over the past few months I've been fighting a bucking /surging condition on my '89 244. I've been bouncing ideas off you guys and picking your brains for info so I want to share the fix with you.

I want to thank you all for good advise, it really helped the reasoning process.

Let me start by giving the symptom again. In cool weather the car seemed to run fine unless you drove for a long time (100 miles or so at highway speeds). In the nice HOT!!!! weather the problem seemed to be anytime you've driven more than a few short miles and the car is warmed up. The car would idle fine but when you tried to accelerate from a stop light it would buck, snort, and surge as if it was running out of fuel and going to die. If you held steady throttle it would level out and seem to run fine. Next time you stopped, same thing. If you parked the car for a few minutes or added gas it would run ok for a few minutes and then start the same old mess again.

I had thought all along that it was fuel pressure but it ran so well early in the morning and in cooler weather so I discounted the idea. I could check the fuel pressure with the engine cold and it was fine ------ or so it seemed. I kept being fooled by all the test because I was not testing long enough.

I pulled the in-tank pump and noticed the strainer sock covered with rust. The light bulb came on. I tested the pump and it was fine. I changed the sock, main fuel filter and then put it all back together. The car ran great for a few days.

Knowing the rust was the problem I found a better fuel tank at a junk yard and it included the pre-pump. Sunday I removed the tank and installed the new tank and pump (after I cleaned everything up, tested and replaced the pre-pump sock).

I wanted to recover the fuel that was left in the old tank so I thought I'd use the intank pump to pump the fuel out into a 5 gallon bucket. I pushed a piece of hose over the tank outlet, connected the black wire to the positive post of a spare battery and the brown wire to the negative. The fuel flow was fantactic even though the fuel was rusty looking. After about 5 minutes the flow slowed down ----- ALOT!!! I shook the tank because I knew it had alot more fuel in it than that. When I shook the tank the flow picked up for a few minutes and then slowed again. So I shook it again and it increased again. I kept repeating the cycle until all the fuel was out of the tank. BTW! I filtered the rusty fuel through a 1 micron filter before I put it in the new tank.

What did we learn:
We learned that the longer the pump ran the more rust was pulled to the filter sock reducing the fuel flow. This reduced flow starved the main pump and caused the bucking / surging condition. In hot weather the fuel volume is very important in keeping the system from vapor locking. In hot weather two thirds of the pump volume needs to be returned to the tank in order to keep liquid flowing and not vapor. Hot weather also causes the fuel to be a touch "lighter/less dense" and harder to pump, also harder to maintain any given pressure. The car ran better in cool weather/early-in-the-morning becacuse it was easier to pump the cool fuel (and the rust had settled overnight). As the fuel was circulated and the tank vibrated from bumps in the road the rust was stired up and started doing it's work and the fuel was becoming warmer. It all seems so simple in hind sight.

I'm sorry for this being so long but I hope it helps someone else understand what happened and how better to diagnose their own issues. I also wanted to post the fix because so often we give advise to people and we never know if it helped or not. Knowing what actually fixed an issue helps us to build a better database in the archives for future use.

You guys are an awesome resource and I thank you for being there.

Tony T.
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Tony Turner 89 Volvo 240DL 257K, 92 Jeep Cherokee 250K, 93 Jeep Grand Cherokee 215K, 1980 Mercedes 240D 360K