M20 5000 RPM Hiccup

[ScottL]

I was going to do a search on this but I couldn't figure out which words to look for, so my apologies if this has been covered.

My 180K-odd mile M20 runs strong for the most part. I've done all the easier maintenance items since buying the car last year: timing belt, cap and rotor, plugs, air filter, fuel filter. Over the winter, I installed an updated fuel tank (from an '88, single main pump) to replace the bashed up, starvy, leaky tank that I had. All the lines up to the fuel rail are new.

Everything is working great except for a nasty cut-out at full-throttle around 5K. This happened before I put the new tank in, too. I originally thought this was due to the pickup problems in my old tank but the new tank hasn't solved the problem. The new main pump appears to be working fine although I haven't tested its output pressure. I also haven't tested the fuel pressure regulator. (I'm thinking about installing a dash-mounted fuel pressure gauge.)

The hiccup does not occur when accelerating hard in a high gear - i.e., if I upshift to 4th at 40mph and floor it, I can reach redline with no hiccup. The problem usually only happens when blasting through 1st or 2nd, when redline is reached fairly fast.

I think this is a fuel delivery problem. Dirty injectors? Bad regulator? Weak pump? Vacuum leak? Does Motronic 1.1 change the fuel strategy under hard acceleration to allow longer injector pulses? Thanks!

[X Mon]

I'd see if you can get your hands on a spare coil. Thought here being that you're not getting proper spark when you have a rapid paced high demand for coil discharge. I had a similar issue on the one iX, but it's been too long now to remember the specifics. I do know it was a mid RPM total loss of power, only when on full throttle in the lower gears.

[O 16581 72452 5]

Search for "AFM", i've posted a few times on fixing the problem related to it.

You can either try another one, or pop your's open and bend the contacts. You'll need silicone to put the lid back on.

[OptimusGlen]

also... the Throttle Position Sensor (TPS) is located on the bottom side of your throttle body, over time oil can seep into it and hose things up. Try unplugging the sensor and see if it still does it, if it doesn't you need a new sensor.

[ScottL]

Coil, AFM, TPS - I'll check out all of them and report back when I can. Thanks!

[Austin]

Yah had a similar issue ended up being the TPS.

[bmwsun]

i have a similiar issue with my m20 i am currently dealing with, i have changed the tps, sparkplug wires, fuses, distributor cap, and the coil and the problem still exists. so i dont know what to do. Sounds like ScottL and i are in the same boat? help

[Austin]

Clogged fuel filter ?

[ScottL]

I think that a clogged filter would result in poor performance across most of the rev range especially under hard acceleration and/or high RPM. I'm seeing a single, split second major dropout - it jerks the car hard enough to knock my (unsecured) gauge cluster loose. (Gotta fix that.)

At any rate, my fuel filter is new. Can't say that it's completely clean, but it's new.

[bmwsun]

ScottL is your car a auto or manual?

[Argie]

i have a similiar issue with my m20 i am currently dealing with, i have changed the tps, sparkplug wires, fuses, distributor cap, and the coil and the problem still exists. so i dont know what to do. Sounds like ScottL and i are in the same boat? help

Neither of you mentioned if you had checked out the crank sensor. When my went out, everything work great while the car was cold. Once warmed up, I started having similar problems.

[bmwsun]

correct i havent changed the crank sensor. where would i find that?

[ScottL]

My car has a manual trans.

The crank sensor is is a small cylinder mounted on a small arm coming from lower passenger side of the engine block. It sits very close to the teeth of the crank pulley and has a wire coming out of it.

I hadn't thought about that being a problem. I thought that a failing sensor would cause all kinds of problems including hard starting. It's probably worth considering though.... thanks!

[RichV]

Definitely look at electronics. Sensors, AFM would be my 1st suspect. You can probably test it with an ohmeter on the right pins and move the trap door through its range. Make sure there is not a dead spot. Or just plug in a buddy's sensor for testing. Cranks sensor would make the car die in most cases, like mine does, unexpectedly, no RPM consistency.

Are you getting any codes? Plug wires that break down will typicaly do it at certain RPM, usualy high RPMs and will feel like a miss or stumble.

[greese]

Swap the #9 fuse. Check the DME relay. Also, see if you have a 173 DME. If you have a 153, that might be the problem. I think 87s' were in transition from 153 to 173.

[bmwsun]

to my knowledge my #9fuse is ok, however i have noticed that my #9 fuse is stainless covered while all others that i have seen are white plastic. any reason for this being different?

[triggrhaapi]

I had the same intermittent issue on my M20 and thought it was a dead spot in the AFM, but it went away after I put an ebay chip in the ECU. Hell if I know why that is.

[greese]

Don't go on looks. Swap that fuse.

[bmwsun]

i cant find another one like it where the numbers match up and the dealer wants $15 for a new one.

[ScottL]

Looks like the problem is the TPS. I tested it per Bentley's procedure (first at the ECU connector, then at the switch itself) and found that the switch is open at full throttle when it should be closed.

So, I removed the throttle body and tried to adjust the switch per the manual. The switch would not close at full throttle no matter how far I rotated it on the mounting screws.

The whole switch is soaked with oil, so I ordered a new one. Can't wait to see if this cures the problem!

I also tested the pot on the AFM. The flapper moves freely but I saw some strange readings on the ohmmeter. The manual is pretty fuzzy on the correct resistance range for Motronic 1.1 AFM's so I'm gonna let that go until I get the new TPS.

(Oh, and I measured the resistance of the crank reference sensor - it was fine.)

Thanks for the replies! I'll have an update in a few days.

[ScottL]

I installed a new TPS and the problem is still there. A weekend at the track tells me:
a) it's intermittent
b) it's inconsistent - coming out of turn 1 at Summit, the engine burped TWICE. 5000 and 5500. sometimes it burps at 4500.
c) fuel flow is still a potential cause but the problem has occurred in a straight line as well as after turning

Soooo, I ordered a new set of plug wires. I need them anyway, so I consider them to be good maintenance. If that doesn't fix it, I'll go after the AFM. If that doesn't work, I'll remove all new parts from the car and drive it off a cliff.

[excellone]

Any luck? I think it is a tired fuel pump or sticky injector/s. Try some fuel injector cleaner.

[bmwsun]

changed crank sensor still same problem......long time ago

my car just decided to stop doing it...problem fixed for me at least, i just replaced a whole bunch of parts as maintence and i guess it fixed itself

[theroundcow]

experiencing similar issues here. car does a pretty dramatic jolt around 3k in all gears eventually but not consistently. now there is a list of refurbished parts and i put my car thru hell. ebay fuel pump like 6 years ago, refurbished e28 injectors, my afm has had the lid off of it for a while now....my plug wires are original, i have copper plugs in there that couldnt be more than 2 years old max...

i know its not the tps sensor, problem persists when unplugged. i dont think its fuel delivery because after the hiccup i can floor it all the way to 7k no problem.

whats this afm problem? seems likely since the lid came off mine and i never found it again and kept driving.

- - - Updated - - -

confirmed afm issue. but all i did was press the contacts down and created a more embedded path....its getting dirty. ill get on that.