Rough running after BBTB refurb...

Wed Apr 05, 2017 3:25 pm

Ok, so the car is a '90 touring 325i - which was turned into a 2.7 by Ant when he was running A-tech. Everything that can be modded has been, although I was suspicious that the 'wrong' throttle body had gone on, (not the big bore jobby).

I've been having a problem with the throttle sticking slightly open, which has been getting worse and worse, so often I'd be idling at 1500 rpm, which as you can imagine is both annoying and not ideal when cruising or slowing by coming off the throttle!!!
It appeared to be caused by the throttle body not quite closing enough - I could pull it closed with the arm on top and the idle would go to normal, then spring back a touch when I let go.

I sent the throttle body off to be refurbished (the guy who advertises on ebay) and to be turned into a big bore if it wasn't already. It wasn't and he refurbed & enlarged it and sent it back promptly - however, this is where the problems begin.

I've not been doing the work myself - the car is in a workshop near where I work.
They bolted the BBTB in place and the car didn't want to fire. They got it running rough by tweaking the throttle stop screw, (which they admitted was never a good idea).
I spoke to the guy who rebuilt the TB and he said 99% it'll be the tps.
I fed that back to the garage, (who I believe have reset the throttle stop), who had a go at resetting the TPS.
It now starts and idles, but still runs very rough, seems very flat through the rev range, not taking off at all after 2000/2500 rpm as before (running a non-standard cam too!).

They are going to look into this further for me and try to get it sorted, but I wanted to ask on here and see if we are missing something obvious?
I thought the TB was normally bolt off & bolt back on and away you go???

We are also assuming the throttle stop screw was in the factory position prior to being removed, as the car was modded and mapped with the old TB setup!!!

All ideas & suggestions welcome please...

Wed Apr 05, 2017 4:13 pm

Just spoke to the guy working on this for me to make sure my info was accurate...
They have re-set the throttle stop back to manufacturer specs, which is where we believe it was before the TB was refurbed/modded.
The car does idle, but seems to raise and drop the revs - holding for a while in between. When driven it feels very flat and underpowered and doesn't seem to come 'on cam' at all.
They've checked gaskets and checked for air leaks several times, all ok.
He's also tried leaning off the fuel mixture to try and rule out things like the temp sensor.
His next step will be to properly test the TPS and make sure that is 100%.

I should also say the car has just had new plugs, rocker gasket, valve clearances done, dizzy cap & rotor checked ok and was running nicely until the TB was removed and replaced, with nothing else done in the interim!!!

Wed Apr 05, 2017 6:02 pm

The TPS (throttle position sensor on the underside) gets filled with oil and starts to misbehave. I seem to remember that a potential fix is to drill a small hole in the underside and spray some contact cleaner through it to see if that sorts it

Wed Apr 05, 2017 7:09 pm

TPS on a M20 is a very simple device, and easily thoroughly checked in a couple of minutes. The idle switch is an enclosed miniature standard microswitch, and will fail to work if its full of oil. The WOT switch is just a crude wiper switch which never/rarely fails, although IIRC, I did once read of a case where the wiper leaf had broken.
Throttle stop isn't going to have any effect on the engine's running under throttle, so why are they playing with that? I'm afraid I get the impression that whoever's doing the work doesn't really understand how these things work.
How'd they "lean off the fuel mixture"? It's controlled by the ECU software. The only thing they can readily 'adjust' is the AFM bypass, which again, is an idle adjustment, and will have next to no effect at WOT.
If the TPS tests don't show up anything, I think I'd be looking at the wiring to the plug and socket under the inlet manifold, which connects the injector loom to the main engine loom. There's a good chance that this wiring would have been disturbed while doing the work.

Thu Apr 06, 2017 5:48 pm

Thanks Brian.
I'm going to try and get to the workshop to have a look myself at some point.
AFM was replaced with a MAF way back when the engine was built!!!
The wiring could be a good shout.
They are usually really good in there, but I know they're under it at the moment and the guy working on mine (around their regular work) - his old man just had a nasty bike crash and is not in a good way!!!