MB hesitation when warm

AdrianH · 05-02-18, 10:17 AM

After about eight years of progressive work (over when I got the car) my MB is now running - in general - very sweetly. There's just one last nagging glitch. When the engine is cold throttle response is smooth throughout the rev range and under all load conditions. However, once it's warm there is what I can only describe as a sharp hesitation when I come back on the throttle at about 1500rpm under load. This doesn't happen while the engine is warming up. Normally I can drive around this but in town it's a bit of a PITA and it doesn't feel good for the car –*it would be nice to get it sorted. BTW the car has a new ISV and the picture is all set up correctly according to Phil Payne's instructions. Th only slight question mark in that is that I'm not sure when the gas analyser I used was last calibrated.

Anyone any ideas?

vagdream · 05-02-18, 10:45 AM

Presume you have checked that the WUR or control pressure regulator is working correctly and allowing the required increase in fuel control pressure when warmed up?

AdrianH · 05-02-18, 10:53 AM

That was my initial suspicion –*unfortunately I don't have the gear/knowhow to correctly diagnose, or how to adjust.

AdrianH · 06-02-18, 09:10 AM

OK looks like I'm going to be taking some readings. Couple of questions – what size connectors am I looking for on the fuel pressure gauge I'll be buying, and can anyone point me in the direction of the various pressures I should be looking for (i.e. system pressure, control pressure cold, control pressure hot etc).

Thanks again!

vagdream · 06-02-18, 07:57 PM

This might provide a few clues;
http://audifans.com/mirror/www.isham.../pressure.html
extracted from the last few pages of Phil Payne's quattro bible.

05-02-18, 10:45 AM	#2
vagdream Grown up member Join Date: Jul 2015 Location: Llandudno Junction Posts: 165	Presume you have checked that the WUR or control pressure regulator is working correctly and allowing the required increase in fuel control pressure when warmed up?

05-02-18, 10:53 AM	#3
AdrianH Grown up member Join Date: Jan 2014 Posts: 218	That was my initial suspicion –*unfortunately I don't have the gear/knowhow to correctly diagnose, or how to adjust.

06-02-18, 09:10 AM	#4
AdrianH Grown up member Join Date: Jan 2014 Posts: 218	OK looks like I'm going to be taking some readings. Couple of questions – what size connectors am I looking for on the fuel pressure gauge I'll be buying, and can anyone point me in the direction of the various pressures I should be looking for (i.e. system pressure, control pressure cold, control pressure hot etc). Thanks again!

06-02-18, 07:57 PM	#5
vagdream Grown up member Join Date: Jul 2015 Location: Llandudno Junction Posts: 165	This might provide a few clues; http://audifans.com/mirror/www.isham.../pressure.html extracted from the last few pages of Phil Payne's quattro bible.