Revs drop when blipping throttle pedal

Post Thu Dec 22, 2016 12:54 pm

I have a '89 / 320i manual, with a b25 fitted.

Recently noticing that when the throttle pedal is pressed a little bit the revs drop and engine feels like it'll stall? If I push the throttle to half way down, there is a brief stutter but then picks up. Bit disconcerting when pulling out into traffic, and blooming annoying when trying to crawl the car into/out of a parking space.

Apart from that initial drop off, the engine runs sweet and pulls fine.

I have checked around the air lines and idler valve and can find no obvious leaks (cleaned out the valve just in case). The injectors were serviced last spring (2k miles ago).

Any ideas or things to try?

Post Thu Dec 22, 2016 1:35 pm

This is as the result of a weak mixture as the throttle is opened from fully closed. The usual causes are inlet side air leaks, worn injectors and/or a badly adjusted throttle position sensor.
You say the injectors are rebuilt and there are no air leaks, so check out the TPS. It's not enough to make sure that it switches when the throttle is opened - it must switch immediately the throttle leaves its stop.
To adjust it, you'll need to unbolt the throttle body from the manifold and turn it over. Make sure the rubber blanking plug on the port for the fuel tank vent system, nearly touching the TPS, is in good order, (assuming you don't have a closed circuit tank vent on your car.)
I've assumed that your B25 was fitted complete with 2.5 AFM and ECU.

Post Thu Dec 22, 2016 2:37 pm

Thanks Brian.

I did swap over a complete B25 engine (including afm), the only part still used from the B20 is the ecu with one of Speedtouch's 325 mild chips.

I need to give it an oil service over Christmas, so will run through the inlet side and check for leaks again. I will also do as you suggest and check the TPS.

Thank you for the advice.

Post Thu Dec 22, 2016 11:29 pm

I learned that even hoses that looked fine when on the car were leaking when removed and examined. I replaced the lot in the end.

Post Sat Jan 07, 2017 9:06 pm

Does it act like this?

[youtube][/youtube]

If so ... I'm on the hunt for a similar solution. I just got through replacing all intake side gaskets as a precaution. Rubber bung looked fine and tested tps which was working perfectly. Vacuum lines look good but I think I'm just going to replace them as a precaution.

Post Sat Jan 07, 2017 9:43 pm

Wack the air mass box and i mean a good wack. I bet it comes back to life. It happened to me in the blackwall tunnel of all places thought I be on the morning travel news. It will only get worse and new or good second hand one is needed

Post Wed Jan 11, 2017 11:37 am

Pulled the inlet side apart over the weekend. Checked and cleaned the TPS and all the hoses, all looked in good condition. Put everything back together and its much improved (even the brakes work a bit better - I also checked the servo line) - still a bit of a hesitation on slight throttle opening, but its defo better than before.

The only thing I really changed was the gasket between the TB and the manifold - I shagged the gasket when I was removing the TB - no spare gasket to hand so used some sealing gum stuff - I'm wondering if a lack of air tightness between the TB and manifold was the primary offender?

Car drives much better than before, thanks everyone for the advice.

Post Sat Jan 14, 2017 7:57 pm

Glad you got it fixed.

Did it act like the video I posted up earlier? If I tried to blip it twice quickly it would stall.

I too rebuilt the fuel rail and replaced all the gaskets on the intake side and was sure it was the TB gasket at first because it looked all perished when I removed it. Replacement was a paper gasket but seemed to fit well. I've got a load of gasket maker but wanted to use OEM parts where possible.

I guess I need to test brake booster vacuum etc then on to spark (already replaced rotor arm) and also need to check altenator ground as I have read this can also cause issues. Then I'm starting to scrape the bottom of the barrel.

Post Mon May 01, 2017 12:39 pm

I'm still having this issue intermittently although is much improved. Does anyone have any other ideas? I ran a makeshift vacuum test (excluding brake booster) and seems to hold pressure well. So far I have:

All new intake side gaskets
Cleaned injectors and replaced o-rings and filters
New air/fuel filter
New plugs
New rotor
New distributor
New HT leads
Inspected TPS (operation and setting appears to be good)
Cleaned ICV and MAF

I think I'm going to blank the brake booster vacuum on the TB now and see if helps (to rule out vacuum leak inside the booster) after that I'm a bit stumped. Anything else I've missed that would cause the issue to manifest only between idle and giving throttle? Otherwise runs fine and idles well. No hesitation going through the rev range.