98 obw cam belt failed now #1 misfire

[wayneb]

Hi All;

Cam belt failed recently, replaced and had misfire on #1 & #2(CEL). Replaced cam belt & idler(failed),coil, wires & plugs. The car idles decent when cold but real rough when warmed up. It seems to run real well at speeds, just poorly at idle when warm.

I have an actron scan tool that is indicating #1 misfire but only get CEL once engine warms up.

Could the cam or crank sensor have been damaged when belt broke?

Or perhaps injector problem?

I know in previous threads others have suggested bent valve but valve train is real quiet and engine runs great under acceleration.

I am going to perform a compression test tomorrow, but my intuition tells me it's something else.

 

Thanks in advance;

 

Wayneb

[idosubaru]

definitely worth making sure it's not something simple but it wouldn't be that surprising for it to have a bent valve anyway. that there is no noise and it still runs doesn't mean much, it might be tweaked just a bit to keep it from seating flush against the valve seat. if i had to guess i'd still guess bent valve.

 

yes the cam and crank sensors are relatively close to the cam sprockets and crank pulley. they're the only electrical gizmo's located directly behind these items, they typically have orange connectors and are very easy to find, the cam sensor is behind and on the upper side of the top drivers side cam sprocket. very easy to see and replace - one bolt and it's the only sensor back there that can read the cam position.

 

i think it runs in 'closed loop' mode when it's warming up, so that's why you're not seeing any check engine light until it's up to operating temperature and goes into open loop mode where it's actually incorporating feedback from all the sensors.

[wayneb]

Hi Grossgary;

 

The thing that baffles me is that the engine runs smoothly when cold/closed loop conditions. would a bent vavle causing misfire only show up in an open loop condition?

 

Wayneb

[idosubaru]

i'm not sure what the ECU reads to determine a "misfire" code but if it's one of the engine sensors that isn't in use until it's up to operating temperature then that would make sense to me...of course the bad cam/crank would as well, i know a few people have had failed cam/crank sensors i'd be interested to know if those codes appear before/after the car warms up. maybe someone will unplug one for you and see! or you could try the same...clear your codes and unplug the cam sensor (easiest one to get to). that still doesn't verify anything if nothing happens, but if the CEL comes on right away that might prove something.

[nipper]

Lets start out with basics, how many miles are on this engine.

 

It has been reported here that subaru ECU judges misfire by a simple comparison. The Crank sensor sends out a signal to the ECU to fire a coil. The coil fires. The ECU monitors to watch that this happens.

 

I have alwasy hoped that it is a bit more sophisticated then that, but aparently not. With a good osscilliscope, one can tell when the misfire happesn in the spark cycle (primary, secondary, voltage leak etc), but the days of scopes and engine analyzers seem to be gone.

 

In open loop, the engine runs off precalibrated settings. It still needs the cam position sensor and crank position sesnor, that doesnt change. When you switch to closed loop, the ECU is running off the input of the O2 sensor.

 

What you may be mistaking as a loop status switch, is just a warm engine vs a cold engine.

 

You can check the Crank sensor for signal both warm and cold, but I dont think that is the problem. Cam sesnosr as an FYI is input for the fuel injectors. If it was a position sensor and the car is running, it would throw a code.

 

Move the ignition wires from 1-2 to 3-4 and see if the problem moves. you may have a vacume leak, DOuble check all the hoses.

 

 

nipper

[wayneb]

Hi Grossgary;

 

Thanks for the ideas.

 

I would think if you unplugged the crank sensor the engine would die. I thought the crank sensor is neccessary for spark. Again with the cam sensor, if unplugged engine should die since it's necessary for injector signal.

 

I wonder if it will behave as I imagine or not? I'll have to let you know what I find.

 

I always thought open/closed loop operation just meant whether the oxygen sensors are being utilized for air/fuel ratio. but then again I learned computer control for autos using GM system, others could be different.

 

Wayneb

[idosubaru]

i'll be interested to see the compression test results.

[Skip]

just a correction to some previous statements

 

Open loop = cold engine ECU ignores the 02

sensor(s) as it is not up to op temp. A/F ratio

determined by the FI coolant sensor.

 

Closed loop = warm engine - all systems monitored and

A/F ratio is adjusted by the ECU as per the 02 sensor(s)

[OB99W]

[...]I know in previous threads others have suggested bent valve but valve train is real quiet and engine runs great under acceleration.

I am going to perform a compression test tomorrow, but my intuition tells me it's something else.[...]

Wayneb, it was you who questioned whether valves could have been bent, and I among others told you it was one possibility. See: http://www.ultimatesubaru.org/forum/showthread.php?t=83636

 

It still is probably a good idea to check compression.

 

 

The thing that baffles me is that the engine runs smoothly when cold/closed loop conditions. would a bent vavle causing misfire only show up in an open loop condition?

Actually, it runs "open-loop" (without certain sensor feedback) when cold, and "closed-loop" when warm. In other words, the ECU uses certain "default" values when the engine is cold; they include things like long injector pulse to enrich the mixture. The rich mixture and other default settings when cold can hide certain problems that become evident when things warm enough for the ECU to go to closed-loop operation.

[OB99W]

[...]It has been reported here that subaru ECU judges misfire by a simple comparison. The Crank sensor sends out a signal to the ECU to fire a coil. The coil fires. The ECU monitors to watch that this happens.[...]

As you know, the crank and cam position sensors generate pulses based on the reluctors passing by them. It's my understanding that for a certain RPM, the ECU expects each pulse to occur at a particular time. When one is "late" (meaning the engine has slightly slowed), it interprets that as a misfire of the previously-fired cylinder.

[WAWalker]

If the timing belt fails on an interferance engine, it is (or should be) standard procedure to check for valve damage.

A compression test is good. A cylinder leak down test is better.

 

The reason I recommend a leak down test......................I worked on a WRX that had been in a crash, timing belt was broken.

Insurance company was paying the bill. They payed for a compression test. Resaults were good compression on one bank, no compression on other bank. They athorized removing the head on the bank with no compression ONLY.

 

Did valve job on that head (bent valves), got the engine back together and it ran OK but it wasn't right. I then preformed another compression test. Results were, good and dead even compression readings on all 4 cylinders. I then performed a cylinder leak down test. Results, 50% leakage on the head that the insurance company wouldn't pay to be removed. Well they had no choice, they had me pull that head.

 

On first glance valves did not appear bent. When the valve springs were removed and the valves rotated they would be seated, rotate about a 1/4 turn and a gap would open up. Pulled valves out, the steams were bent in two places, an S shape bend.

 

I know you said yours is competly quit when running, (this car did have some valve train noise).

 

Anyway.....................rule out bent valves, before spending to much time chaseing gohsts.

[wayneb]

Hi All;

 

Engine has 215,000 miles with head gasket./seals/valve job/water pump/timing belt and idlers replaced at 185,000. The lower idler pulley failed which caused the belt to break

 

Thanks for all the input. It appears that the compression test will really tell me where to go from here!

 

I've replaced plugs with NGK and swapped wires with problem remaining on #1 cylinder.

 

I think my biggest problem is I don't believe a valve is bent cause it runs so dang good other than idle, probably have to recalibrate my way of thinking in this matter!

 

WAWALKER Thanks for the info on bent valve inspection. I have a feeling I'm pulling engine/head to check for how many valves are damaged. would have been my next question to forum.

 

I'll update y'all sometime tonight/tommorrow after compression check.

 

Wayneb

[wayneb]

Hi All;

 

Well it's a no brainer! Compression check showed following: #1-90psi,#2-150psi,#3-185psi,and #4-185psi.

 

I should have the engine out this morning, all that's left to do is pull hood, motor mounts,bellhousing bolts,and flywheel bolts.

 

My next question is should I replace bent valves, have heads rebuilt, or purchase rebuilt heads?

 

Any other suggested repairs/parts to replace whilst engine is out?

 

Thanks for alll your help;

 

Wayneb

[Skip]

Howdy Wayne,

No need to pull the hood as they stand straight up.

 

I tie mine off to the rear tiedown.

 

The factory prop rod can be removed and used

but the rope does not get in the way while wrenching.

 

You can find all the obvious replacement parts

in one of the

other many threads on this proceedure.

 

One sometimes missed is removing the oil pump and

applying lock tite to the rear screws.

[WAWalker]

Rebuilt heads are quite spendy.

 

You say that it had a valve job ~30k miles ago. With over 200k on the engine I wouldn't spend the money on remaned heads, or a complete rebuild of your heads. I would probably just replace the valves that are bent.

 

Hard to say for sure without seeing the engine after tear down.

[wayneb]

Hi All;

 

Pulled heads and found interesting observation, The exhaust valves on cylinders 1 & 2 had marks on the as though they made contact with the pistons. These are the cylinders that were causing misfire issues, especially #1 which had more prominant marks on exhaust valves.

 

I'm going to pull valves off for inspection tomorrow, I suspect that the exhaust valves on mentioned cylinders will be bent! Overall rest of engine looks great!

 

Wayneb

[idosubaru]

as with most things it's not always complete broke or perfect...there's lots of in between that can happen, this is your case.

 

replace the bent valves, i would not get new heads or remanned heads. i'd send them to a machine shop myself and have them checked - for flatness and the valve seats, making sure the bent valves didn't wear the seats unevenly. or i'd ask, i'm not really familiar with what could happen in this kind of situation.

 

glad you got it figured out.

[wayneb]

Hi All;

 

Found the following: both intakes and 1 exhaust valve bent in #1 and one exhaust valve bent in #2. Interesting in that the valve face was bent on shaft but shaft was staight on all the valves.

 

after reading some of comments about bent valves, I expected shafts to be bent but now that I think about it, the way they bent makes more sense.

 

Just awaiting parts to rebuild.

 

Wayneb

[WAWalker]

As far as my comment about the WRX valves bending the shaft.......................I did leave out the fact that the valve steam is longer, or at least there is more steam exposed between the head of the valve and the end of the valve guide. That is why they will bend the steams. Not the case with the 2.5L DOHC engine.

 

I was just trying to make the point that a cylinder leak down test is better than just a compression test. But in the case of the 2.5's a compression test will probably always show low compression if a valve is bent.

 

Sorry to mislead you.

[wayneb]

Hey Wawalker;

 

Not a problem, just never seen a bent valve before and your comment was the only one I think I saw about it.

 

OWB, ordered parts from 1stsubaru.com yesterday afternoon with overnight shipping and got them today!

 

Great place to buy parts as far as price and availablility. I used them two years ago when I replaced head gaskets. Also great customer service.

 

I can't recommend them enough!

 

Wayneb

[wayneb]

Hi All;

 

Replaced bent valves and re assembled engine, put in car but when started ran terrible! Did compression check and #2 cylinder would not hold compression. At idle vacum wobbled and had dead miss.

 

I pulled engine and checked #2 exhaust valve that was replaced and saw no problem.

 

Anyone have any ideas?

 

Wayneb

[nipper]

Did you do a wet and dry test on all the cylinders? If so we need to know the numbers.

 

 

nipper

[wayneb]

Hi Nipper;

 

Dry #1,3,4 185 to 190 psi. #2 maybe 10 to 15 psi.

 

I suspect a bad exhaust valve seat on valve that was bent and replaced in that cylinder but can't see anything to indicate that.

 

Wayneb

[nipper]

Silly question, are you re using head gaskets every time you take the engine apart?

 

nipper

[wayneb]

Hi Nipper;

 

Replaced head gasket upon reassembly. Haven't reused gasket assumed that you can only torque it once, is that true?

 

Wayneb