Stall/cutout under load

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Lock
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Stall/cutout under load

Post by Lock » Wed Jun 05, 2019 10:36 pm

Hey guys, my car has developed a bad stall under load. It will have power for a moment, then cuts out for a second, then power for a moment and so on, but only under mild to heavy load. It's not engine speed dependent. It's like anything beyond 1/4 gas pedal will cause it. Currently it's undrivable, getting up to speed on the freeway is nearly impossible. Have swapped some parts and checked wiring and everything, but all looks in order.

Revving without load is ok, engine will rev up to ~6000 then level out at an unsteady 4000rpm with foot flat on the gas.

Due to it just cutting out (not gradual) I think it's either the AFM, like high airflow reading is faulty, or the ECU itself. Anyone experienced this or have thoughts?
1979 528i 5-speed conversion G260/6

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Re: Stall/cutout under load

Post by tschultz » Thu Jun 06, 2019 5:02 am

Are you sure battery connections are all good?

Clogged fuel filter?
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Re: Stall/cutout under load

Post by 528i-1981 » Thu Jun 06, 2019 5:52 am

Had similar symptoms with water in the gas. Drove me nuts because everything else looked to be in order.
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Re: Stall/cutout under load

Post by Mike W. » Thu Jun 06, 2019 12:38 pm

I actually had something very similar long ago on an 80 E24. Over half pedal and it would cut out. Turned out to be a broken wire going to the FI temp sensor up by the stat. Pull on the wires a bit to test. My break was hidden by the boot, but eventually I found it. It was the torquing action of the engine that would make or break the connection. Probably not your problem, but worth a check.
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1981 528i, 3.6, Recaros and more. Project
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Re: Stall/cutout under load

Post by Lock » Thu Jun 06, 2019 12:50 pm

Battery connections are good, I replaced the battery recently but can clean the terminals just to make sure. I know L-Jetronic is sensitive to power and grounds.

Also just replaced both the fuel filter and fuel pump a few weeks ago as I thought it was a fuel issue, but no change (getting noticeably worse actually).

It's almost like the engine is hitting a rev limiter, it's so on/off that I don't think it's a mechanical analog problem. I noticed when in the garage and foot flat on the gas and it drops down to 4000rpm, the fuel pressure gauge I have installed drops to near zero and then back up as the engine fires again. Our cars don't control the fuel pump though, just a simple relay with the AFM flap, but I will bypass everything and link the fuel pump direct to battery + to see if that's it. Maybe the fuel pump line is being bumped during certain AFM flap travel?

So either the fuel pump is cutting out by chance when the engine is under load, or the ECU is opening the injectors fully (explains the low pressure) but the engine isn't firing. The ignition isn't 'smart', just vacuum/centrifugal advance.

I'll do some diagnostics and see what the various components are doing. Hooking up some 12v bulbs to the coil, fuel pump and such will let me know what is getting power and what isn't.

I will check the temp sensor too - mine is down under the manifold I think (integrated with the mechanical idle control valve). My engine does physically move on the mounts so I did wonder if it was just a wire disconnecting when the engine twists. My joints have no corrosion but a lot of the insulation is brittle.
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Re: Stall/cutout under load

Post by 528i-1981 » Fri Jun 07, 2019 4:40 am

Maybe try disconnecting the idle switch on the throttle if you haven't already. If the switch is closed at high RPM the ECU would think you're in coast-down and cut the fuel. Peter's L-Jet FAQ says this feature was added in 79.
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Re: Stall/cutout under load

Post by PatinaBeforePolish » Fri Jun 07, 2019 1:15 pm

Could the connections to the idle and WOT switches be crossed or the switches loose? I accidentally crossed mine which caused a similar situation.
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Re: Stall/cutout under load

Post by Lock » Fri Jun 14, 2019 12:26 am

Yes I thought it could be the throttle switches too, so I disconnected them - no change. I disconnected all the sensors in the engine bay in fact barring the AFM, no change, still cuts out on and off under load.

I suspected the AFM so I took that apart as well - found very worn tracks so after looking up NLA or expensive parts, I cheated - I moved the circuit board up slightly, so the wiper was on fresh track.

Image

After adjustment

Image

But even after that with nothing plugged in except the AFM it's doing it - I still haven't proven the AFM is okay, it may have some other invisible fault but I don't have a spare and replacements are pricey for just a hunch.

I'm going to connect indicator lights to the injectors, fuel pump and coil + lines to see if one of them is losing power which will narrow it down. I'll do that this weekend, and I suspect as Mike said maybe it's some old wiring breaking the link as the engine moves.

If none of them are the culprits, maybe my ECU has somehow gone bad. Will report if I find anything useful.. I was kinda hoping to go to 5erWest in Portland July, but only if it's reliable again. We'll see.
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Re: Stall/cutout under load

Post by Mike W. » Fri Jun 14, 2019 11:38 am

I noticed when in the garage and foot flat on the gas and it drops down to 4000rpm, the fuel pressure gauge I have installed drops to near zero and then back up as the engine fires again. Our cars don't control the fuel pump though, just a simple relay with the AFM flap,
Now I can't say why it's happening, but fuel pressure should remain relatively constant. That sure looks like your problem from here. I'd do a temp hotwire (emphatically temporary) on the fuel pump so you can determine if it's a switching problem or a pump problem. If anything fuel pressure should increase some as manifold vacuum goes down, but you might not be able to tell in the driveway.
Mike W.



1980 528i, 3.5 euro, 5 speed conversion
1981 528i, 3.6, Recaros and more. Project
1998 328is, quick and efficient, but not satisfying
2000 528iit, Vacation mobile/wife's grocery getter

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Re: Stall/cutout under load

Post by Lock » Mon Jun 17, 2019 10:13 pm

So I'm still investigating, but so far I have wired the fuel pump directly (fused) and it still cut out. The tach needle doesn't drop, which means the distributor is still giving a signal and the ECU and coil is receiving that signal. As it happens under load open-loop it's as if the fuel map has holes, but as we don't have a 'map' maybe one of the sensors is way out of spec causing the whole system to freeze.

I've ordered a set of 12v LEDs and I'm going to wire up various parts to watch what is losing power.

I've also got a second old broken control relay (injector+fuel pump relay unit) which I'm going to solder the contacts closed, and also hook lights up to its lines internally. See what it does and if I can learn anything more.
1979 528i 5-speed conversion G260/6

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