Fuel pressure question, E9 versus E34 535 -91

[Johan]

back with another question, sorry to bug you all....

I am running my new 535 -91 engine with the old e9 pump. It's been working fine for a few years but now I have a problem that I am trying to figure out. The car jerks sharply at low RPMs with constant gas but runs "ok" at higher revs and while accelerating.

A guy here said it *may* be fuel pressure related so I looked up the pressures.

The two values I found are :

L-Jetronic & Motronic & Motronic 1.1
3.0 +/- 0.06 bar (system pressure)
2.8-3.2 bar (system pressure at idle)


E9 31-32psi
31 pound/square inch = 2.14 bar

Seems to be a fairly big difference between these?

So, a few questions;

- should my engine have run at all with this difference?
- are the values correct?
- is it easy to swap pumps to one with higher pressure?
- if you have made the same conversion, which pump did you use and were there any problems mounting it?

Will take it to a garage Monday to measure the pressure, don't have a meter for it myself.

thx,

johan

 

[AndyM]

Johan --

The pump doesn't determine the pressure -- the fuel pressure regulator takes care of that. A stock CSi fuel pump is adequate for the pressures you listed assuming you are running the proper regulator for your conversion. If the pump is failing, there are numerous external pumps you can install in place of it - I'd source a newer Bosch or Walbro in-line pump in place of the stock unit if it needs replaced. It could just be a clogged fuel filter, too.

 

[velocewest]

A 3.5 engine uses a 3.0 bar fuel pressure regulator, so the pump needs to be capable of maintaining 3.0 bar (about 40psi) at the fuel rail. Pressure will drop a little below that as revs rise.

You can make a pressure test gauge from a lawn irrigation pressure gauge and a brass tee fitting. It'll set you back about $10 at the hardware store.

Your problem doesn't immediately bring to mind a fuel pressure problem, since the car runs better at when accelerating and at higher revs. I'd start by checking for vacuum leaks.

 

[Johan]

Thanks guys,

I think I will try and get a new pump then. There is no harm in that I guess and it can't be too expensive?

I'll change the pressure regulator too just in case and every vacuum related hose I can find even if they don't seem to leaking.

rgds,

Johan

 

[velocewest]

Johan, if you're looking for vacuum leaks you need to check over the entire intake track. The hoses are not commonly the problem; more likely would be a cracked intake boot or a leaky gasket on one of the joints on the intake manifold.