Intermittent miss

[hershel]

According to noise in the intake this is some kind of a valve hangup. Runs fine til engine
warms up and then problem is intermittent.
Any suggestions?

[britannic]

Could you describe the symptoms in more detail i.e. how many cylinders seem to be missing, noises etc.?

[hershel]

Sorry for the delay, here is some info I should have provided in the first post.
This engine is supposedly a 1988 and came from a salvage yard. Compression tests show a max and min of 450 and 320 and an avg of 388psi. Oil consumption is about a quart/2000. Mileage is 20mpg with a 350 automatic xmiss. This engine was a replacement for a 5.7 in a 81 chev C10. Mileage since installation is about 4000.
I noticed a noise in the intake (that's where I think it's coming from) some time ago. It was very low in intensity and of short duration and seldom happened. All of this has increased the last couple of weeks but to my knowledge there hasn't been any unusual metal to metal sounds. In my opinion the frequency of the noise indicates the problem is in one cylinder.
I previously said this happened after engine warmup but I have since noticed there is a slight engine unbalance after startup and a less than smooth airflow into the air intake. This may last a short time and both problems disappeare simultaneously.
Assuming there is no lubrication problem I would think this problem could be solved with some kind of cleaner, provided the engine is disabled while doing this.

[britannic]

That's quite a variation on those results. Check out this thread: Compression Test (http://forum.thedieselpage.com/cgi-bin/ultimatebb.cgi?ubb=get_topic;f=2;t=002118) . Could be a valve, or lifter. Another test you could do is to pressurize each cylinder with compressed air (though the glowplug holes) and check for air blowing out of the manifold or coming out of an adjacent cylinder (remove all glowplugs).

[hershel]

Not much has changed in the problem-I have started this on numerous occasions and let it idle. One time I noticed a sort of grey smoke coming out of the passenger side exhaust during the problem. This smoke lasted probably 4 or 5 secs and quit when the valve problem (or whatever it is) stopped. I have noticed a decided unbalance in the engine, a couple of times, during the proboem.

I ran a compression test, thinking I might get lucky and find something. The compression has improved since installation of the engine. The low was 420 with a high of 460. I removed the valve covers but didn't see anything unusual. Not sure what the next step is.

[britannic]

How many miles are on the injectors? Did you try some fuel treatment to clean them? Might be an injector hanging up...

[hershel]

Injectors and pump were replaced before I installed the engine but that doesn't guarantee there isn't one bad. I can't say I've ever had a bad injector but I'm sort of hung up on this noise in the intake thats associated with this. Would an injector be more than just that cylinder not firing?

I noticed when I removed the valve covers that they had been installed using the regular silicon type gasket sealer. Doesn't GM use an
aerobic or anaerobic type gasket? The same stuff was used on the pan which means someone has had this engine apart for some reason.

[britannic]

Thinking along the lines of an intermittent problem, lifters come to mind. When you get the problem again, use a mechanic's stethoscope (or appropriate long screwdriver) and try and track down the source to help with the diagnosis.

[hershel]

I wonder if some type of cleaner might do some good here. Quite some time ago I think an Amsoil Foam was used on an engine but I don't know the details and I can't locate that post using the search function.

I put a can of engine cleaner thru this, the kind you add, run for 5 min, then drain and replace the oil and filter.

[hershel]

I decided that to solve this problem we need to start tearing things apart or get some way to record some engine parameters during the problem. To start, I came up with a device (maybe kluge would be a better name)to monitor the exhaust air flow. This consists of a box which is open on the ends and has a shaft, mounted with bearings, in the bottom of the box. Attached to the shaft is a plate which is mounted with it's surface perpendicular to the exhaust air flow which will rotate the shaft. A throttle shaft position pot,obtained from a salvage yard, is attached to the end of the shaft to indicate shaft position and provide a restoring torque for the system since it has a fairly stiff torsion spring. the pot is offset slightly so the zero deg mechanical stop is provided by the shaft and plate instead of the stop in the pot. One box end was shaped to fit the exhaust pipe and the other end to fit the air cleaner intake. The necessary voltage and signal conditioning was added to get the desired output voltage Vs shaft position. A pulse was taken from the crankshaft pulley to synch the pot output with crankshaft position while monitoring the output on a scope. The scope display is recorded on a VCR because it has a large storage capacity.

The data from the engine while idling is probably not terribly accurate but detecting large changes was the goal. I'm sure measurements are effected by a large system time constant. Also the plate area is being rotated away from being perpendicular to the air flow-not sure how or whether this effects the force the air flow exerts on the plate. Under normal idling conditions the pot output shows a shaft position which rotates between 15 and 65 degs during an engine cycle.

In 1 Hr running time the problem happened once and was brief as usual. The data shows no air flow in the exhaust for about 120 deg of an engine cycle. In fact one could hear the plate hitting the mechanical due to the lack of air flow. This continued for a few revs and then returned to normal flow. I imagine if I had been standing up near the engine instead of at the rear I would have heard the same type of noise I've heard before. During this time a lot of krud was coming out of the exhaust which appeared to be a grayish type of soot rather than what one sees and smells from an overfuel condition. Since this is the only data recorded during the problem I can't say the whether it always gives this type of indication or not.

Anyone seen this type of weird operation?