88 5kq - Intermittant Hard Starting - FIXED! I think

[SJ]

I think the cause of the '88 5kq Intermittant Poor Starting When Cold' is
fixed.
Main Problem was narrowed down to air leak at a fuel injector insert.
Secondary problem was that the idle mixture was at the edge of closed loop
operation.
Original problem post at
http://www.audifans.com/pipermail/quattro/2006-September/112781.html

New fuel injector inserts and O-rings installed.
New fuel injectors installed - old ones had lousy spray pattern, 160K miles,
and 18 years on them.

New problem appeared, the O2 sensor voltage was not swinging during idle. O2
voltage reacted ok when revs were higher than 1800 RPM.

Murphy made his presence known. The 90 100q dropped its full load of
pentosin.
Now I aint got no car that runs!
Replacing hydraulic X-cap O-rings solved the problem.

Well, what the hell . .might as well fix the 100q leaking oil pan. Three
days later, that was done. Good cleaning, acid and zinc phospate etch, and 3
coats of POR 15 did the trick for the oil leak. Fiberglass cloth was used in
the area of the pinhole leak.

Well, what the hell . . now that I'm an expert on oil pan leak fixes .
.might as well do the 5000q oil pan also.
Pinhole found under the rust. Six days later (rainy weather) the job was
done.

Ok, now back to the O2 sensor problem.
I finally figured out how to remove the idle mix plug.
The plug DOES have a steel disc at it's bottom. The drill will stop at this
disc. No need to measure drilling depth.

The screw used to remover the plug needs to be modified for removal to work.
Drill used was 3/16 = .1875
Screw was 1.0" long
OD was  0.138"
ID was 0..105
Screw end was ground off. The screw threads were 1/3 filed off on two sides.
The screw now resembled a flat bottomed tap.

Air bonnet is removed.
Screw was screwed into the plug.
A mid sized nail puller was used to lever out the plug. A block of wood over
the fuel regulator prevented marring of the aluminum. Worked flawlessly.

I dont have a T handled long Allen. I had a normal 3 mm Allen, so I tried it
out. It works, kinda short - but you can turn the screw about 1/4 turn at a
time.
I made sure that the mixture screw could be turned. I turned the screw 1/4
turn, then turned it back to original location.

The engine was started and warmed up.
DVM connected to O2 sensor.
WTF!    DVM is swinging back and forth!!!
Starting to rain - will finish this another day.

Today, I hooked up the DVM, then started the engine.
DVM starts at  0.6 volts, slowly drops down to 0.2 volts and after 4 minutes
of normal idle, the voltage starts swinging as it should - 0.1 <--> 0.8
volts
Perfectly Normal!!!

My analysis:
I think the mixture control was at a point where it was at the edge of the
operating window for closed loop operation. The basic A/F was just a bit too
lean to get the system into closed loop operation.
With a lean mixture and a leaking O-rings at the fuel injector inserts,
starting sometimes would be problematic.
Why sometimes?  I think some of the insert O-rings let in more or less air
depending on temperature conditions. The O-rings were old (18 years), hard
and brittle. Any temperature shift or mechanical shifting would make them
leak. Then the engine would not start. Engine catches then dies.

As for the O2 sensor "problem", when I tested the mixture screw after
removing the plug, I set the idle mix adjust into the operating window
instead of on the edge of the window where it was before I touched the
adjustment screw. The mixture was now in the operating range for closed loop
operation.

I disconnected the O2 sensor to put computer into Open Loop operation, and
attached a DVM  to O2 sensor leads.
Then, while watching the DVM,  I adjusted the idle mix screw for a little
richer mixture.
This adjustment is twitchy. It does not take much to make a dramatic
difference in voltage.

The idle now is much smoother than ever before. The combination of new
injectors and a proper idle mixture eliminated the lean miss condition(rough
idle) that I have had for ages now.
In the past, I noticed that sometimes the engine would idle better than at
other times. I always thought this was just dirty injectors and would add a
bottle of Techron to the gas.
Now I realize that what was happening is that the engine would slip into
Open loop operation now and again, and because the basic idle mixture was on
the lean side, I would detect a lean miss(rough running) at idle.

How did I cover the idle mix access hole? I whittled a piece of cedar shim
stock to fit the hole, and tapped it into the hole. Works kinda like a golf
tee, which I didnt have.

SJ
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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