parts interchange- fuel pump relay 300sel vs 420sel
 

Does anyone know if I can use a fuel pump relay from an '88 420 SEL on an '88 300 SEL? They look exactly the same and they have changed the 300 sel replacement from the original 9 pin to a 10 pin just like the 420. I am having surging/decelerations at all speeds with variable RPM's at stoplights. While on the freeway at 70mph it feels like I'm suddenly hitting a stiff breeze for a second or two, and then the car recovers and continues at same speed. I was going to start with this, then move on to OVP etc if it doesn't work.

by part numbers they are not interchangeable. they may actually energize the pump, but are not correct. do they show different rpm ratings? that is usually one difference. that is the value at which the engine 's rpm is lmited

they should say right on it 6 zyl and 8 zyl, i think the 8 has two more pins for the rpm input maked with "k" or something like that i dont remember exactly.

I'd do the ovp first.

Thanks for the replies everyone. Maybe you are right, I'll start with the OVP. I've got the original with 1 fuse on top on an 18 year old car, I guess they've all gotta go sometime.

Only other thought I had: I just replaced the brushes in the blower motor today. It was immediately after this job that the surge/deceleration started happening. I have a 126 so I accessed the blower from the passenger floor, not from the engine. Is there anything I could have disrupted from there that might have caused this?

vacuum line?

126 surge
 

go back and look at the o2 plugs and make sure that you didn't disturb they are down on the pass side front floor.