88 5kq - Intermittant Hard Starting

[SJ]

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.

Here is a list of observations:
1. Cold start - engine will crank, start, and then RPM's fall off and engine
stalls. Stepping on gas pedal has no effect.
    - Trying to start again results in no start - engine wont even catch.
    - Come back a few hours later, and engine starts right up

2. Cold start - engine cranks, starts, RPM's start dropping . . .playing
with gas pedal keeps RPM's up . .after a second or two engine maintains
RPM's on its own. Engine works fine after this.

3. Hot start - engine shut off after a trip and left sitting for an hour or
so. Outside temp was 90, car in full sun.
    - crank engine, starts, runs for 1/2 second or so, then dies . . like
running out of gas.
    - try starting again, nothing, wont even catch
    - Come back a few hours later when outside temp was much cooler, and
engine starts right up.

4. Engine runs fine under WOT conditions, plenty of power, no fuel
starvation noted.


Diagnosis:
First Bitch - the FA drives me crazy! When looking up the coolant
temperature sensors, the 88 5kq lists the bottom sender(MFS), but not the
top sender. The 100q lists the top sender, but not the bottom sender.
When you look up both part numbers in the "TPS-where used" window, both
senders will be listed for both cars.

I'm suspecting the MFS temperature sensor, thats the bottom sender between
spark plugs 1 and 2. The MFS sensor feeds the CIS control unit so that it
can adjust the Differential Fuel pressure Regulator(DFR) for proper fuel
mixture. Either the electrical connection is not tight, or the CIS half of
this sender is intermittant resulting in a lean mix for starting.

If this was a fuel related problem, like low pressure, I would have an issue
at Wide Open Throttle . no?

If this was a fuel pump check valve problem (leaking > vapor lock), this no
start issue would be a constant problem when starting hot, no? And engine
would always start up fine when cold, no?



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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