Very lean and limited RPM at speed



This engine is a replacement and has yet to run correctly so I cannot say this condition was caused by an event. 1991 325i Convertible engine in a 1988 325e chassis.

The condition is that the engine freely revs to redline in 1st - 3rd gears. Once into 4th, or 5th, and therefore higher speed, it bogs down at about 4500-5000 rpms. Does not miss, just will not rev. Speedo also does not work. It was suggested there is a MPH component to DME (I know there is an input) and that my speed sensor was bad so I replaced it. A quick trip to the dyno (only convenient way to test) revealed extremely lean condition (~ 18).

I am now digging into AFM, O2, TPS and fuel pressure. Does this condition ring any bells and are there any suggestions on how to proceed?




These are classic bad AFM symptoms. Get a known good AFM and go for a test drive.

On a number of occasions, we’ve strapped a fuel pressure gauge to a windshield wiper and T’d it into the fuel supply line to the fuel rail. Then when your car behaves poorly, just look at the gauge to confirm that supply pressure is ok. The fuel pressure gauge kit at Harbor Freight includes a handy T connector. So all you need is 6’ of fuel line to make it work.

Normally for fuel line you have to get high pressure stuff, not the low pressure line that they sell at the local autoparts store. For the purpose of this test tho, the low pressure fuel line is fine.


Thanks Ranger.

I have a good AFM (passed Clark’s 944 test). I will say that it did NOT pass the air temp test (short which apparently causes lean mixture). Off to test the unit currently installed in the car.

Will drop by Harbor Freight for a fuel pressure gauge.



I went and found the Clark 944 test and it’s a nice write-up. By “nice” I mean it sounds right. The test in our Bentley is for the old AFM, but the 325i has the new AFM which Clark correctly points out that the new one has a different test procedure. It was the 944 guys that taught me that, I can’t take any credit for figuring that out.

But Clark’s test won’t detect if the AFM has lost it’s calibration, or if some well-intended person has intentionally messed with it’s calibration by turning the spring-dial. Since your symptoms are classic AFM problems, I’d go find a known good AFM and spend 30min trying it out.

No offense, but someone gave you bad scoop and you wasted your time swapping out your CPS because the symptoms don’t much support it as the problem. I’m now giving you good scoop, so you might as well give it a try.

I read at some point that temp is a relatively weak variable in the fuel mapping. If that’s the case, a problem with the temp sensor wouldn’t cause your severe symptoms.


I’m no expert. But on my 9V reference I got 0.5V at minimum and about 8.5V at max. Seems close. TPS checked out too.

I’m afraid I have fuel pressure/regulator or injector issues.



At about 4500rpm, the AFM is maxed out and the DME uses only engine & intake temps and rpm for fuel maps (adjusted by learned adaptation) if it sees the WOT switch in the TPS closed. The TPS is the first thing I’d check and I’d test for it operating properly at the DME connector.



The issue turned out to be the Fuel Pressure regulator. Installed an adjustable regulator with gauge and problem solved. Still need to go back and replace OEM unit.