Re: 89 F150 stalls at idle / low RPM's
On 19 Jul 2005 20:11:43 -0700, "krobinett"
>Well, it appears that the multiple symptoms that I described were
>caused by multiple problems. After driving around for a while, the
>truck started to stall again at freeway speeds and even died at a
>stoplight. The idle was still solid, but it was running rich and
>I bought a computer code reader ($35 at Walmart) and a code 33 (EGR
>Valve not closing properly) popped up during the KOER test. I replaced
>the EGR Valve and it seems to be running much smoother. But, I haven't
>had the opportunity to get it over 45 yet. One good thing is that the
>new EGR Valve has quieted an engine knocking that was happening durring
>acceleration. It's not gone, but it is MUCH better.
>Thanks again for your help. I'm not even close to being a car guy...
>this forum is very helpful. I'll continue to post updates/questions if
>anything more happens.
More often than not, the EVP sensor on top of the
EGR is bad instead of the valve itself. It tells the ECM
exactly how far open the EGR valve is at any given moment.
The EVP sensor rarely results in a fault code as it is just
barely in the adaptive range of the ECM but, far enough out
that the function of the EGR system is inadequate. This
makes the position of the EGR near the edge of
acceptability. The EGR valve was probably OK if you could
disconnect the vacuum line w/o noting a change in idle
quality. If you have a source of vacuum, you can apply
vacuum manually to the valve. If it then stalls the engine,
it is probably OK. An EGR valve that is not opening far
enough can cause both a limited rich condition and
Your rich condition may be caused by a bad fuel pressure
regulator as they had a fairly high failure rate in that
vintage. If you pull the vacuum line and find fuel, it is
definitely bad. A fuel pressure check should show about 35
psi idling. When you open the throttle, the loss in
manifold vacuum should result in an increase in fuel
pressure to 40-45psi. I've seen a completely failed
regulator result in fuel pressure over 90psi which will
quickly result in a failed fuel pump or other problems. If
you rev the engine and suddenly close the throttle to create
high manifold vacuum, you should see the fuel pressure drop
to 15-20psi. You can, of course check this with a vacuum
pump and guage along with a fuel pressure test guage.
Normally, if the regulator fails, the fuel pressure will go
high and not drop when vacuum is applied. In my own f150
w/5.0L, I saw fuel mileage drop to less than 9mpg when the
fuel pressure regulator failed on a trip - it was typically
in the 17-19mpg range on the same trip.
Another common item that can cause a rich condition is the
ECT sensor (engine coolant temperature). It tells the ECM
the engine temperature. A sensor with an incorrect value
can tell the ECM that the engine is warmer or cooler that it
actually is. This will result in a rich or lean condition
depending on which way it is out of spec.
Another place to look for a bad or erratic idle on those is
a vacuum leak between the upper and lower intake manifold.
They were known to occasionally suck a gasket especially
where someone did not properly torque the upper plenum. You
can usually find this by spraying something like WD40 or
carburetor cleaner along the joint with the engine idling
and listening for a change. Some folks like starting fluid
(ether) but, you can also damage the engine if you get a big
slug of ether into the engine thru a leak. You can also get
yourself blown up if Murphy is upset with you on that day.
If the upper intake has ever been off, check the small
plastic vacuum lines behind the manifold. They are super
easy to get pinched and broken when tightening the upper
intake. This can also warp the casting a bit. Since those
lines are for the EGR, the may play havoc with that system
as well as causing a vacuum leak that affect engine