88 80Q, Runs rough upon starting

SJ syljay at optonline.net
Tue Nov 14 15:41:19 EST 2006

> From: "Eric Huppert" <dragracer at netstep.net>
> Subject: RE: 88 80Q, Runs rough upon starting
>If car is allowed to sit 5-10 minutes after shutting off, (or even on a
>start) upon restart car fires, then starts to stumble and drop RPM's.
>Feathering gas pedal, etc, doesn't help. But, discovered that full throttle
>will get the rpm's up, but when let off it still runs rough. It clears up
>a minute or two, but something's not quite right>

You may have a lean mix at idle. Engine starts up ok, because cold start
valve provides extra gas. Engine starts running rough after cold start valve
shuts off.

Test for Lean Mix at Idle
After you get the engine running and warmed up, test oxygen sensor voltage.
See if it is fluctuating between 0.2 and 0.9, or close to that.
If no fluctuation, rev engine to 2000 RPM and see if voltage fluctuates. If
You may have a lean condition at idle that prevents system from going into
closed loop mode. The mix is too lean for the EFI system to compensate.
But at higher RPM, the air leak is insignificant compared to volume of air
rushing in, so EFI system works OK.

Easy way to test this is to position propane tank hose in front of the air
intake at the front grill.

(I use my barbecue propane tank. I have a manual adjust pressure regulator
which makes it convenient for adjusting gas flow. The propane tank shut off
valve is too coarse for fine tuning.
If you have a fixed pressure regulator, you may need to add a valve after
the regulator to control gas flow.
Cut the tank-to-barbecue rubber hose after the regulator. You can fix it
later with short length of tubing and hose clamps.
Insert 1/4" ID (0.390" OD) copper tubing into hose.
Attach about 5 foot length of 3/8" vinyl hose to other end of the copper
tube. Use teflon tape if the fit is a bit loose.
Attach about 18" of 1/4 copper tubing to other end of the vinyl hose to use
as your probe.
You can sweat on a cap at the end of the tube and drill a small hole in the
cap for better control of the gas.)

While watching O2 sensor voltage, add propane till O2 voltage starts
If Voltage starts fluctuates, then the engine has a lean mix for some
reason. The propane gas is compensating for the lean condition.

To test for vacuum leaks, use propane gas.
I use a 1/4" copper tubing attached to a hose, with a curve at the end of
the tubing so that I can reach the fuel injectors.
I pull the connector off  the Fuel Pressure Differential Regulator (limp
home mode) to make sure the computer wont be changing A/F mixtures while you
are troubleshooting.
Disconnect the O2 sensor lead from harness, and connect DVM to O2 sensor
Note: The computer applies 0.5 volts to this connection until system goes
into closed loop mode. This voltage will over-ride whatever the O2 sensor is
putting out, and you wont be able to see any changes based on Propane
enrichment. Thats why you need to disconnect the O2 lead from the harness.

Test this procedure with hose at the air inlet so you get a feel for it -
adjust the meter for a low range if meter is not auto-ranging.
With DPR disconnected, the O2 sensor will read about 0.004 V . . .real low.
Adjust propane until the meter reads about 0.5 V or thereabouts.
(At this point, if you attach the O2 sensor back into the harness, you will
see the engine working normally. O2 voltage will be swinging between 0.2 V
<-- > 0.9 V.)
Now remove the propane hose from air inlet and start checking for air leaks
around the engine while watching the DVM.
Test all air connections and hoses with propane while observing the O2
sensor voltage. The O2 voltage will react very quickly when you richen up
the mixture

Pay particular attention to the air injector seals/inserts. They may seal
when cold, but open up when warm . .or vice versa. Put propane tubing tip at
the base and around each fuel injector to test.

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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