High RPM bucking problem

[Velocewest]

Motronic car, chipped ECU, stock ignition. Revs to the rev limiter in 1st and 2nd, in 3rd it starts coughing and pinging above about 3500 rpm when accelerating with the throttle open more than about 50%. Why does the gear I'm in matter? Urgh. It's been doing this since I pulled the transmission last weekend. I cleaned everything well before reassembly, but maybe some crap is on the flywheel teeth or the speed or ref sensor. Bentley doesn't seem to cover this type of problem in the "Driveability" troubleshooting section.

Other facts -- fresh WR9LS plugs, just adjusted the valves today, no vacuum leaks I can find. Engine has about 15k on a good rebuild, the spark plugs that came out were uniformly light brown and clean.

Thoughts?



[Edit by Velocewest on [TIME]1107750803[/TIME]]

[T_C_D]

Sounds like the engine is running out of fuel. There is alot less load on the engine in the earlier gears.
Todd

[Velocewest]

[QUOTE="TCD"]Sounds like the engine is running out of fuel. There is alot less load on the engine in the earlier gears.
Todd[/QUOTE]

Hmmm... I did notice today that I could hear the main fuel pump, which is new. Maybe the pump is giving up. I have a couple spares, I'll stick one in and see if that helps. Probably should check the filters too.

Thanks Todd!

[booker]

O2 sensor..

Try unplugging it and running open loop. If the hesitation stops, you may have your culprit.

If it is during low fuel levels, you should definately suspect the in-tank pump.

[Velocewest]

Thanks, I'll try the O2 sensor too. No in-tank pump in this car, it's a '75 530i.

[Skeen]

I bet it's the fuel pump; I had a similar problem with mine and that fixed it.

[Velocewest]

Stopped by Home Depot at lunch and grabbed a 100psi pressure gauge and some fittings ($10). Plumbed it into the supply line and fired it up. Once the engine is warm, it shows about 30psi (2 bar) at idle and drops to about 25psi as the revs climb. Pulling the vacuum line off the FPR doesn't help. Also noticed that it has a 2.5 bar FPR... That ain't helping either.

Looks like I need to swap the filter, then test the pump. And get a 3.0 bar FPR. Time to head to the u-pull-it... VDO makes a nice 1.25" 100psi pressure gauge for about $20, think I'll order one of those to replace my cheapo and leave it in the line.

Thanks for the advice, all.

[Velocewest]

ARRGH! New 3.0 bar FPR, new filter, ALL new fuel lines, new fuel pump. Still can't get more than 2.5 bar at the rail. Thought I was going to lunch the damn motor schooling some IDIOT in a ratty ass e24 who refused to surrender the left lane or go more than 60 mph, of course until I decided to go around him on the right.

How the hell can I still have low fuel pressure? Some big booger in the hardline? Could this be a bad ref sensor? Bad TPS? Why is it only in 3rd gear and above? Should I just get a 3.07 diff so I can go 100 mph in 2nd and call it good?

[kendogg1]

Just out of curiosity, what's fuel pressure supposed to be at? My buddy had the smae problem with his 355 E30, anx it turned out to be the coil. sorry, didn't mean to throw another part into the mix.

[Tjn182]

So wait... a loud fuel pump = a dying fuel pump? Because mine has always been noisey.

[FirstFives Dictator]

[QUOTE="Velocewest"]ARRGH! New 3.0 bar FPR, new filter, ALL new fuel lines, new fuel pump. Still can't get more than 2.5 bar at the rail. Thought I was going to lunch the damn motor schooling some IDIOT in a ratty ass e24 who refused to surrender the left lane or go more than 60 mph, of course until I decided to go around him on the right.

How the hell can I still have low fuel pressure? Some big booger in the hardline? Could this be a bad ref sensor? Bad TPS? Why is it only in 3rd gear and above? Should I just get a 3.07 diff so I can go 100 mph in 2nd and call it good?[/QUOTE]

Remember the 3.0 is NET to manifold pressure. At idle it might be around 2.5bar (gauge) which would be around 3.0 absolute. But when you rev the engine the gauge pressure should climb towards 3.0bar. With no engine vacuum the pressure should be 3.0bar gauge

[Velocewest]

The gauge will jump to 3.0 bar if I pull the vacuum line. It does NOT seem to go up as I rev, though -- pressure drops down several pounds. I'm going to mount the gauge on a longer chunk of hose so I can run it up through the cowl grill and see it while driving. Then I can see what is happening with fuel pressure when the engine starts crapping (and probably rear-end someone in the process of staring at my hood instead of down the road).

[FirstFives Dictator]

[QUOTE="Velocewest"]The gauge will jump to 3.0 bar if I pull the vacuum line. It does NOT seem to go up as I rev, though -- pressure drops down several pounds. I'm going to mount the gauge on a longer chunk of hose so I can run it up through the cowl grill and see it while driving. Then I can see what is happening with fuel pressure when the engine starts crapping (and probably rear-end someone in the process of staring at my hood instead of down the road).[/QUOTE]

Sounds like the regulator is ok. Check voltage on the pump to make sure it's good (if low, it will deliver pressure but not at volume). If low, may ground or relay problem.

Also make sure level sender screen is clean (I'm assuming this early model has no prepump)