Hi everyone,

here i go again.

Over the past few months I have been on a trek to solve an idle issue. There are other posts in this quest.
I thank all for the advice and help over this effort.
I am so close now. But there is a bit of a stumble/oddity that continues to elude me.
Need some help figuring this one out.

To start this part of the repair:
Idle was erratic, not stable.
MPG was 12-12.5 mpg city 19.6 mpg freeway
Ran well on freeway.
Had a stumble pulling through the gears - momentary hesitation - like ignition
Idle drops to 450-500rpm during warmup recovers to 700-750 after hot


Replaced parts / work done:

injectors cleaned
top end refresh done
front end (TB/WP) done
fuel pump / check valve and filter replaced
replaced the fuel pump check valve recently.
FPR (both are new)
replaced the Temp II sensor (before the O2 sensor)
replaced the O2 sensor (after the Temp II sensor)
found small unmetered air leak and repaired.


Testing:

fuel pressure is a solid 2+bar running
fuel pressure immediately starts falling off when car is shut off, drops to zero after 10 minutes.
FPR (both hold vacuum for 20 minutes)
FPD holds vacuum for 2 hours
no fuel smell in FPR/FPD hoses
replacing the Temp II sensor restored the idle. (Replaced before the O2 sensor).
replaced the O2 sensor after the Temp II sensor (marginally bad, did not respond quickly enough - idle became 'better' after replacing O2 sensor)
idle CO is set to .45-.5V
deacceleration value works properly
AFM (Barn door) tests within specifications
the ECU test okay per the FWSM process (or maybe the Blue book)
smoked tested the engine vacuum system

Observations:

starts well cold, cranks a bit (overnight - most likely due to no pressure in the lines).
starts well warm, idle is good or not depending on what the coolant temperature is.
starts well hot. Idle is good.
The idle is good after startup.
idle is good until the temperature reaching just above the first (bottom) line on the temp gauge. Idle is lower during this time.
Idle is good after reaching operating temperature, or slightly before.
after the car reaches operating temperature idle is good. Maybe a bit high but solid with no hunting
idle raises when the AC is on.
Stumble running through the gears is gone now.

What ideas do you have?

 

What year and fuel system are we working with?

 

My bad. Forgot to include that vital information.

It is a 1981 4.5L with L-jet FI.

Thanks

 

please post a picture of the engine bay

 

Thanks for the reply.

Here is the picture

MY1981 4.5L L-jet

 

upside down air cleaner filter,
no hot post cover ,
coil cover missing
Green wire looks new is it a Bosch wire or URO version?
Eductor port misrouted SB going down between the two aft legs of the spider
Washer spout cap missing

 

Hi,
Thanks for the input.

air cleaner is easy to fix
Hot post cover in a bin in the garage - looking for opportunity to install
Coil cover is needed
Green wire is the Porsche (Bosch) version. FOE installed (thanks Sean Ratts for supplying the replacement)
Attempted to locate the port plumbing but was not able at the time -- will look at it again.
Washer spout and the spout cap need replaced - used part is in the same bin as the hot post cover. awaiting opportunity to install.

My 928 is a project. I am enjoying the journey as well as the finish.

On the idle issue:

This idle issue is a nagging thing I've been chasing for over a year. The idle is very close to being correct but not there yet.

I am leaning towards the ECU being a bit off as the reason for the idle issue.
Have tested the FI wiring harness; all appears correct.
Alternator is tested and output is very much within spec.
Ignition switch (electrical portion) replaced
Starter is newer Bosch hi-torque type
Engine grounds are remade with carbon grease used (in fact - all grounds on the car are remade)

Thanks for the help. Appreciate it.