Barely-running '89 90q

[Bernard Littau]

Hi Charles,

My 86 4kq had similar symptoms at one point.  Problem turned out to be 
the ECU temperature sensor located by the thermostat housing.  Yours may 
be different on the 90q, but on the 4kq the sensor has 10-15 K ohms 
cold, and it goes to about 2K ohms warm.  If the sensor has too high of 
resistance, the ECU makes the mixture too rich. 

I think the 89 90q is a CIS-E like the 4kq, but it could be different, 
so check your Bentley.  Mine mentioned a 15K test sensor used to get the 
4kq going when cold.

I disconnected the sensor and put three 4.7K resistors in series in 
place of the sensor.  All three were in series to start the car.  By the 
time I got to my mailbox 1/4 mile down the road, I went to two in series 
to have the car run well.  A little further along I went to one 4.7 K 
resistor, then half of one (two in parallel for about 2K ohms).  A more 
smarter person would have wired a 10K pot into the passenger compartment 
and used it to dial in the best running, like in the old days with a 
manual choke :-)

A disconnected circuit appears as infinite resistance, so disconnecting 
the sensor will not make the situation better.  You can try shorting the 
circuit (zero ohms) once the car is warm to see if it runs better.  
Better would be to put 1-2K ohms of resistors in place of the sensor 
once the car is warm.  If it runs well while warm with the resistors and 
not well with the sensor, its a good bet the sensor is bad.

Best,

Bernard Littau
Woodinville, WA

DePenning, Charles [EPM/MTN] wrote:

>Well this has really turned into a stumper.  Some of you may recall the
>problem I'm having with my 221k mile '89 90q- in short, it runs like it is
>missing on at least one if not two cylinders.  Starts like crap, runs like
>crap when warmed up. This behavior started suddenly at highway speed, with
>no indication of this before.  No power, and the exhaust smells pretty rich.
>Here's what I have done:
>Checked spark- OK
>Checked compression- OK
>Checked TDC on cam compared to flywheel- OK
>Checked fuel system pressure(s)- OK
>Checked fuel pump delivery rate- OK
>Checked O2 sensor operation-at least attempts closed-loop
>Checked cat- not plugged, but I hollowed it out anyway
>Checked for vac leak- as well as I could, hoses look ok
>Pulled Codes- 4444
>Pulled out and tested injectors- thought I had it here.  All five really had
>a bad spray pattern.  Like little needles.  So I ordered up five new
>injectors from Blau, put 'em in (and cleaned up a lot of crap under the IM,
>gave the IM a nice coat of paint, valve cover gasket, painted that
>too...sure is pretty now).  Spray pattern looks great now, so I installed
>them with confidence that this would solve all of my problems.  But noooo.
>Still runs the same, maybe a little worse.  Argh!  I'm getting thin on ideas
>now.  Would a big vac leak cause this behavior?  I am just baffled by the
>fact that it ran great for a good long while and then suddenly crapped out.
>
>Please help me keep my sanity!
>
>Thanks in advance-
>
>Charles DePenning
>'89 90q
>'84 4kq
>'83 5kT
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