Misfire at idle, some hesitation, some codes...

[ryan.hess]

'95 3100, has a random misfire at idle. I have not noticed it at any rpm above idle. You can really feel the misfires hit, and the RPM on the tach drops slightly everytime it misfires. Seems to not happen so much when cold (at all?). Digital EGR codes are lit up (history). So is a transmission range switch error (history).

Previous owner replaced lots of things, probably trying to get rid of the same misfire. Coils are new. Plug wires are *probably* new. Not sure about plugs. I tried to pull a plug, and the boot just would not budge no matter how much I pulled. I might need to get some wire pullers or something.

Exhaust smells like fuel and you can hear the misfire clearly.

With the randomness it almost has to be ignition related. Current plan of attack is to check plugs to see how new they are. If they're new, the plug wires are probably new too. (they say champion on them, and look fairly new.) If everything else is new, it makes me think maybe a crank sensor problem.. Thoughts?

[ryan.hess]

Ahh, got a new one.

MALF CODE 17 CAM SENSOR ERROR

[IsaacHayes]

Cam sensor error would be fuel related. SFI motors use a cam sensor to know when to squirt fuel for SFI instead of just firing one whole bank at a time. If the sensor goes bad, it defaults back to MPFI sorta, and will run, but not as smooth.

I'd check the wires to the cam sensor, and replace it. Make sure to get one that is the same year as your motor (95). (if you have a chiltons/etc repair manual, it will show you how to replace it without taking off the top manifold, and reroute the new one differently)

The miss-fire problem does sound like spark, but if you got a code for bad cam sensor, i'd fix that first.

Might want to pop the EGR off and clean it, and clean the EGR tunnel that goes into the intake just to get rid of the code. Also check to see if the EGR isn't sticking open.


If you still have problems after the cam sensor is fixed, and you know your plug-wires are good, I would suspect one of two items (both spark related):
1) Crank Shaft Position sensor
2) Ignition control module (ICM)

When these fail, they do so when they get hot. I've replaced the ICM before and it was the Crank sensor twice. Go for the crank sensor first, and don't get autozone one, they are cheap and will fail in a year. Go to o-riellys/etc. The ICM is expensive, and so far its not been the problem when I've had missfires. It's been the crank position sensor everytime. It will get worse and then you'll drive and it will miss fire /cut out bad, then eventually (like when summer rolls around) it will just stop and you wont get spark and cant re-start the car. Let it cool for a while, then you can start back up and go until it heats up again.

The crank sensor is a PITA to get to, but you need a 10mm deep socket, some extensions on a 1/4" ratchet and do it by feel and you can get it. Jack up the passengerside, take off the wheel, and put your arm through either the bottom or through where the tie rod comes out. I cant remember which.

[ryan.hess]

Plugs and plug wires are definately brand new. Don't recommend the "champion" brand wires. Pulled the clip off the wire trying to get it off. Doesn't seem to mind not having the spark plug clip though...

Generally speaking, what are the symptoms of a failing crank sensor? Will it misfire? Cut out? Just die and not start? If I start it tomorrow morning and it misfires cold, would that rule out a crank sensor?

I can't imagine a cam sensor causing a misfire?

[IsaacHayes]

You can re-crimp the "clip" on the wire. Strip the wire back and re-crimp it on, making sure the boot is on the wire of course first, then pull it back down.

Bad Crank sensor will misfire, cut out, and die and not start. How badly they are worn out will determine which if not all will happen in time. Generally when cold they are ok, but they could possibly act up even when cold. I've had one be fine then cut out and the car start back firing, then resume normal operation. Had one stop sparking and cut out on the highway and kill the whole motor. Waited a while for it cool off, and resumed, it cut out a few times but kept going. Had one cut spark at WOT then hiccup/backfire and I shut it off for 10mins, started up fine and went on my way. So it can be random and progressively get worse.

Since you say only at idle you have the problem, have you ruled out a vacuum leak? Is the LIM fairly new? The LIM can not only leak coolant and oil, but vacuum leak as well.

The cam sensor I can only see as if it's giving really weird signals and then causing the computer to squirt fuel at really the wrong time, would cause a miss fire/etc. Generally it would just run rougher and you get less mpg. But I've never had one go bad so I'm not sure how it would really affect the appearance of the running of the engine.

I guess you need to make for sure it's a misfire and not just running rough from the cam sensor fueling being possibly messed up?

[ryan.hess]

Well I did some testing...

Tried unplugging the cam sensor to see what that did... - nothing.
Tried grounding the coil towers one at a time to see if I could isolate one cylinder... no. Two cylinders had a significant effect on idle note (the outer most two, IIRC), the rest were just about equal.

The upper plenum has some red RTV on it, which concerns me a bit... Could be a vacuum leak or stuck open EGR...

[tractorman]

.

Put some silicon grease on those spark plug boots- it'll make yer life lots easier next time. Also, I've run into several of these 3100's that had bad corrosion where the spark plug wires snap onto the coils.

[IsaacHayes]

RTV on the UIM? yeah I'd investigate that... perhaps a rigged repair job from previous owner?

[ryan.hess]

latest update - EGR valve and passages were clogged with carbon, likely causing the 3 EGR codes...

Still working on cleaning the carbon out of the manifold passage....

How much work is it to remove the UIM? Just need plenum gaskets and EGR gaskets?

[tractorman]

Yeah, that's all you need, and you'll have to take the coolant hoses off the throttle body if it has that setup- I don't know if they started that in '95 or '96. It's not a big deal to remove the upper intake. Both throttle body hoses (if yours has them) hook to that heater hose pipe that runs around the engine. It'll be a lot easier to clean out your egr passage with the UIM off. I cleaned mine out with a coathanger wire, and it wasn't too difficult. I've always left the throttle body attached when I've pulled the UIM, but some folks unbolt it and leave it there.

[ryan.hess]

Well, this weekend I'm taking the UIM off. Here's some pics of the RTV damage:

Driver's side:


Pass side:


Also, found some other damage under the throttle body bracket. On top of the throttle body /neck area, there is a rubber grommet thing which leads to the PCV valve and something else on the other side of the engine. Apparently mine was broken off - likely causing a vacuum leak, although I'm not sure it's enough to cause my miss, and it certainly doesn't explain the RTV. (so even if replacing it does fix it, the plenum's still coming off.)

[ryan.hess]

Question - does this engine really have 2 crank sensors like Mitchell repair says? The yellow/purple wire "low res" sensor going to the ICM had it's wiring melted to the exhaust shield. The wire appears in tact, but the split loom tubing is gone, and the wire is discolored. So what's the deal? Is one for fuel, one for ignition?

[bszopi]

One lo-res (7x) for standard ign timing.

One hi-res (24x) for ign timing at low RPMs. This one allows for a smoother idle and other stuff like that.

[ryan.hess]

Well, I put it back together, fixed those broken vacuum lines, and the EGR, and it runs worse. Now it's stumbling bad from a stop.

btw, the RTV was there to fix a previous leak - I think the previous owner replaced the UIM for some reason, and left on a piece of the gasket, and put another gasket on top of that. So instead of taking it off again, slathered some RTV on it. That was corrected before I replaced the UIM gaskets apparently.

[ryan.hess]

Tested fuel pressure - 39psi @ idle
Tested new EGR for vacuum leaks to the outside world - none

Changed TPS and reset computer... Ran pretty well for a while, then started putt-putting and rhythmically missing, then back to random misses.

Unplugged 3x sensor from ICM - verified that the 3x "low res" crank sensor is only used for spark. Might need to replace that as my next step and/or doublecheck wiring. I need to rescan for codes.

Oh - also, it is idling at 1100 in park.