Sprint 2003 955i Idle Hesitation Problem

stephengrenfell · May 24, 2020

Hi All,

I have a 2003 955i ST with 54K km with an idle problem, hesitation on opening the throttle below about 2.8K revs, regularly pops on throttle roll off, throttle sync impossible (screws seem to do nothing).

This started with me thinking a throttle sync problem. So I removed the tank and connected a carbtune vacuum gauge (using T pieces in the hoses and so with the air box on). Syncing was difficult (I’ve done it many times before) and on opening the throttle I noticed quite a bit of carbon build up below the butterfly valves.

So I took of the throttle assembly and injector assembly. Cleaned out the throttle assembly with carburettor cleaner (gently scrubbing with an old tooth brush were needed), and making sure the throttle sync screw holes were cleaned out too. Idle hoses are new too.

Reassembled everything with new O rings (sync screws, injector top / bottom, position sensor) and replaced the connection pieces as they were rather hardened. Check the throttle cable for suitable play. Adjusted the throttle position sensor to get 0.60V in TuneECU. Started the bike and performed Reset TPS in TuneECU

But it’s still the same.

TuneECU says Idle stepper motor at 7, AFAIK it is supposed to be about 30. Setting it to 30 in TuneECU the engine revs to 2K.

Took of the air box and watched the if the stepper motor rises and lower after switch off, it seemed to struggle (maybe). So I replaced the idle stepper motor and air temp sensor .

Still no joy.

I took off the throttle assembly and checked the connection piece O ring was the right one (there are 2 part numbers listed one is thicker than the other) and the O rings do seem to be doing their job.

I also removed the sync screws to make sure I’d assembled them in the right order (screw, spring, washer, O ring). I noticed that when removing the screw the now new O-rings just came out with the screw whereas the original O rings always stuck in the hole and needed to be coaxed out. Ah I thought, so filled the hole with a dob of silicone (like it says in the Triumph workshop manual).

But nope no change…

Since taking off the throttle assembly and injector assembly over a month ago. I’ve done about 20 hours of riding / commuting so plenty of time for it to readapt, and have several tank full’s of fuel run through.

I’m at a loss.

Anyone seen this before ?.

Thanks.

Mark Balmer · Jun 18, 2020

Have you tried running a tank of fuel through the bike with injector cleaner in the fuel. Has worked for me in the past on other bikes. I use STP. £4, gotta be worth a try.

freck · Jun 18, 2020

Sounds to me like it’s running either too rich or too lean.
They are set with the standard map, to be quite lean for emission purposes, but if you’ve had all the sync screws out you could have screwed up the base setting.
The throttle bodies are designed to have number 1 as a datum (and the setting screw should be sealed)and the other two are balanced from that datum point, so if no. 1 isn’t set right it’ll bugger it all up. I think there’s a no.1 base setting described in the Triumph workshop manual if I remember correctly.
Many idle problems are also attributable to air leaks with the TB base gasket or the idle air control circuit so it’s worth making sure they’re all good too.
I’ve also found before (and have read others’ experiences) that using T pieces with your gauges gives bad/false readings making synchronisation really difficult. It’s best to connect directly to the spigots on the TBs, and shim out the throttle cable linkage to achieve a steady idle of about 1100rpm, or as I do, adjust the cable at the twistgrip.