88 5kq - Intermittant Hard Starting

SJ syljay at optonline.net
Sun Sep 3 14:08:04 EDT 2006

Update on the Problem:
Original post 21 July 06

> 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.

Once started, Engine runs fine 100% of the time. It will start fine 99% of
the time. Sometimes, it will stumble on start up when cold.
Problem isolated to a fuel issue. Tested for spark when engine would not
When engine would not start, using starter fluid got it started.

Newest detected symptom:
Oxygen sensor wont work if revs are under 2500 RPMS. Same problem with a new

Parts swapped out with my 90 100Q
Coolant temp switch
ISV valve - idle valve
DPR - Differential fuel pressure regulator

Parts tested:
Cold start valve works
idle switch works . .was rebuilt before

Systems tested:
Fuel delivery system tested - all test fuel pressure test points agree with
factory specs


The oxygen sensor issue and intermittant no start/stumbling might be an
excessive air symptom (vacuum leak). I didnt find anything obvious, nothing
cracked or defective.
I set up a propane leak test. Used my barbecue tank with adapted regulators
and feed lines.
My first test was to see how far off my idle speed mixture was. I connected
the propane to the fuel vapor carbon filter port in the boot.
I wanted to see if I could compensate for any leak, and get the oxygen
sensor to work.
I disconnected the O2 sensor lead from harness, and connected it to a DVM.
Engine started, ISV valve disconnected, DPR disconnected.
I turned on the propane and slowly increased the flow.
DVM started out as a low negative number(very lean) and started going up as
I increased the propane flow. With fine tuning of the valve and propane
pressure regulator, I could get the O2 sensor to float at around + 0.5volts,
which is Stoich Air Fuel mix. OK, theory works so far.

I then did the same experiment on the 100q audi. Hmmmm, starts off the same
. .O2 reading as negative number.
Increasing the propane level changes DVM reading . . .I got it to settle at
+ 0.5 volts.
I was hoping that the O2 reading would be close to 0.5 volts with the DPR
disconnected. But, I think the injection system is set up to run on the lean
side if the electronics fail (limp home mode).
What I did notice though, was that I need less propane to get the DVM to
read 0.5 volts. This confirmed that I had an air leak on the 5kq audi.

I then switched to "propane leak detector mode" -- long hose and thin copper
tube probe at the end.
DPR and ISV disconnected.
O2 sensor disconnected and hooked up to DVM.
I found the air leak, a small one at the #3 fuel injector. No other leaks

Note: Using the DVM and the O2 sensor is a very effective method of finding
a vacuum leak. The DVM reacts to the smallest changes in A/F mixture
changes. This is much easier than listening for RPM changes.
I probed all over the engine and only found that one leak at the #3 fuel

Maybe its possible that the #3 fuel injector seals shift and develop a
larger leak under different temperature conditions?
Maybe another injector also leaks under "ideal" conditions and this makes
the A/F too lean for engine to start?

I want to take out all the injectors, check them out, and replace all the
seals. Car has 170k on it. Original injectors.
I also want to remove the fuel injector inserts as these have seals also.
Is the job easier if I remove the intake manifold?
Can the intake manifold(old rounded style) be removed without taking off the
head? Bentley does not help here.
Is there a kit for all the seals and O rings?

I've read horror stories about dropping the insert plastic parts inside the
I need some BTDT advice and tricks for this R&R job.


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