Greetings folks, I have a strange idle behavior to describe.
Sometimes it gets rather low & lumpy but the one perfectly
repeatable (and odd) behavior is:
When the car is warm and I’m driving it, and very slowly,
easily come to a stop, the idle settles down to about 700
and is normal.
However, when I stop harder, after the car drops below say
10-15mph, the idle settles down to 500, and a few seconds
after I’ve come to a complete stop, the idle peaks up to
750, settles back down to 650 or so, and remains lumpy until
I press the throttle again.
I also connected an OBDII scanner and found that the O2
sensors spike up the fuel trims the instant that this RPM
spike occurs.
I’m wondering if this isn’t the crankshaft position sensor
returning wonky readings (or no readings), so the ECU is
using default timing tables that might not quite match
reality? I don’t know quite what to check here. No trouble
codes; but the ignition coils are brand new, and the fuel
pressure is right on the mark. Once the engine is at speed
it runs normally. And, if it was an air leak, I’d expect the
idle speed to be consistent but lumpy rather than these odd
dips / spikes.
Any thoughts?–
Andy - '96 VDP - '99 M-B S500 - '99 M-B S420 - '99 LS400
Raleigh, North Carolina, United States
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–
In reply to a message from andyman32 sent Sat 22 Jan 2011:
When I had similar problems it turned out to be the contacts
for the plug underneath the throttle body. Areas I’d
consider looking at are:
Cleaning the MAF sensor
Checking for air leaks at the jubilee clips for the ducting
between MAF and Throttle body
Cleaning the throttle body
Resetting the butterfly in the Throttle Body which often
sticks over time as it changes position with heating and
cooling. loosen the screws mounting it to the spindle and
retighten in closed position.
The crank sensors can become faulty.
The engine temp sensor can go wrong.
Has anything been done to it recently? Service perhaps?
Plugs changed? Mine ran really erratically with NGK plugs
and was fine with champions. Was anything undone and not put
back properly?
Regards
John–
The original message included these comments:
Greetings folks, I have a strange idle behavior to describe.
Sometimes it gets rather low & lumpy but the one perfectly
repeatable (and odd) behavior is:
When the car is warm and I’m driving it, and very slowly,
easily come to a stop, the idle settles down to about 700
and is normal.
However, when I stop harder, after the car drops below say
10-15mph, the idle settles down to 500, and a few seconds
after I’ve come to a complete stop, the idle peaks up to
750, settles back down to 650 or so, and remains lumpy until
I press the throttle again.
In reply to a message from twojagsv8 sent Tue 25 Jan 2011:
Hi John, moving through this problem (which may have made it
worse, or compounded it, instead of correcting the lumpy
idle I set out to fix originally) I have:
Replaced all 6 ignition coils
replaced idle air control valve
removed and thoroughly cleaned throttle body
conducted fuel pressure test (which subsequently broke a
welding joint in the fuel rail - happy day, that one!)
removed & cleaned crankshaft position sensor
replaced all 6 fuel injectors
That was mixed together with refreshing the shock absorbers
on all 4 corners, ball joints and tie rod ends, and
troubleshooting an uneven braking problem which led me to
replace the calipers and flexible hoses.
At this point if I were to just continue to spatter new
parts about the engine hoping something did it, but without
any credible direction one way or another, I would replace:
crankshaft position sensor (this one is only 2 years old
anyway)
fuel pressure regulator
throttle position sensor
all four O2 sensors
possible MAF sensor
Any one of those could be the culprit, or none of them may
be. However, even finding the best deals, that’s a good
$800-$1000 of parts. It may still be worth it as the idle is
starting to get pretty erratic, as is startup. I’m fairly
certain the TPS is knackered. But I don’t know if it’s
causing the rest of this weird lumpiness; I suspect not.
Even if the potentiometer were returning erratic, inaccurate
readings to the ECU, the idle still should not settle below
its correct value unless there’s an air or fuel problem.–