88 5kq - Intermittant Hard Starting

SJ syljay at optonline.net
Tue Sep 26 11:23:34 EDT 2006

> Symptoms:
> 88 5kq - no turbo - NF engine
> Symptom: Intermittant Hard Starting under cold and hot conditions.
> Majority of the time, the car starts up fine . .cold or hot.

Symptom details and diagnosis are here:

I replaced the fuel injector plastic inserts and sealing washers.
I put in new fuel injectors. The original fuel injectors all had different
spray patterns. 165K miles and 18 years is a long, long life for a fuel

The idle is now much improved, very smooth compared to the idle with the
original injectors.
Starting is different now. With old injectors, the engine would start after
barely turning over . .. waa . vrooom.
Now it takes a bit longer to start . .waa ..waa .. waa ...waa .
roooom   --   similar to how my 100Q sounds during starts.

I checked for vacuum leaks using the propane method. None found.
The O2 sensor still will not work at idle. It works fine at higher RPMS. DVM
shows idle mixture is lean at idle.

I did an experiment to see what would happen if I introduced enough propane
to overcome the idle lean condition.
I disconnected the ISV and DPR.
I added enough propane into the intake to bring DVM to 0.50 volts (stoich)
I reconnected the ISV and DPR . .. had to restart engine to reset the
computer, otherwise ISV wont work right.
With the added propane, the oxygen sensor was working correctly at idle .
.fluctuating back and forth around 0.05 Volts.

Preliminary Diagnosis:
I suspect the idle mix is too lean to get the computer into closed loop mode
when engine is idling. The DPR cannot compensate enough for the lean Basic
Fuel Metering.
All fuel pressure tests were in spec.
Fuel Distributor works fine, no binding.

The only conclusion that I can come up with to explain the lean Basic Fuel
Metering idle mixture is:
1. idle mixture setting screw worn slightly at the contact point causing
less lift of fuel distributor control-plunger, and/or
2. build up of fuel varnish on the fuel distributor control-plunger barrel
fuel slits. This would close up the slits slightly and reduce fuel flow to
the injectors at idle.
Injector flow tests at idle and WOT check out ok - - very small
disprepancies between the new injectors, which means the barrel slots are
all similar in flow.

The solution seems to adjust the idle mixture (3 mm Allen) in order to get
the O2 sensor working at idle.
Bentley says to use a CO meter to set the idle mixture. I dont have a CO
meter. Research shows that CO meters are highly unreliable (Phil Payne). I
think Phil uses ColorTune or something like that.

I was thinking of adjusting the idle mixture by watching the DVM. When the
O2 sensor starts working (swinging), the mixture is on the low (lean) side
of the DPR's control range. Maybe adjusting another 1/4 turn or so to richen
the mixture will get the mixture centered in the DPR's control range.

Any other opinions?

85 Dodge PU, D-250, 318, auto
85 Audi 4k - - sold but still on the road
88 Audi 5kq
90 Audi 100q

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