Valve train noise ? missing/backfires now on acceleration

Sorry, I haven't been able to spend the time on this until recently.

I warmed the car up and did compression testing on all the cylinders.
I started on the even cylinder side of the engine as that is where the noise appears to be coming from and did that side first
and then did the odd side. Here are the results.

Cyl. press
2 - 185
4 - 185
6 - 187
8 - 185

1 - 190
3 - 185
5 - 185
7 - 190

The missing and backfiring is very evident when revving the engine and also when trying to accelerate. This problem started the same time as the valve train noise.

I haven't done the 'ground a nail placed in the distributor' test yet with a vacuum gauge.

Devon made a comment made about 'a dial indicator with a magnetic base is in order'. Can someone point me to more information on how to do that?

Thanks to everyone for their comments so far.

Len

 

Is the motor backfiring out thru the Carb or the Exh.
This is a important detail!

 

Mine was doing the same thing. Round cam lobe. The rocker was still moving but it was opening less than the others.

 

Exhaust

 

That is exactly what I am thinking...

 

Yes, I have seen it.. compression tests are about sealing, if the lobe on the cam is not completely round you can fool the test. It takes surprising little valve opening at cranking speed to fill the cylinder adequately.

Only making noise hot can be an indicator that the failed lobe has just now started eating into the base circle enough that it is affecting lifter preload. TA alum heads swell about .012 to .018 when they go from room temp to operating temp and that maybe decreasing lifter preload enough to get the noise now. That's one possibility, and it fits your symptoms.

I would do the cylinder shorting test as described, to isolate which pair of cam lobes it may be. Then your looking at visual inspection.... you don't have to pull the cam out, the damage is going to be very evident on bottom of the lifter.

Any cylinder shorting test will be more dramatic if the engine is under a slight load.. block the wheels, and have an assistant put the car in reverse. Now raise the idle speed to 1000 via the carb screw, while your assistant holds the brakes, and the wheels are blocked. Use a heavy duty test lead, with one end clamped to the battery negative, and touch each nail with the other end.

JW

 

You could also idle the motor and pull one valve cover off one at a time to look at the push Rods.

Good Cam lobes and lifters will spin once every time a valve opens and closes, and those push Rods that do not spin or only partially spin will be quite easy to spot.

 

"110 volt" house current is "110 volts" RMS, about 170 volts peak. Similarly, 277 and 480 are RMS numbers, with the peak voltage somewhat higher.

"12-volt" ignition secondary voltage is typically ~8000 volts, but potentially in excess of 30,000 volts. The only good thing about that is there's little amperage.

Be glad that 8000+ volt jolt doesn't ground through your pants-zipper on the fender.

 

Yes. A running compression test will show a flat cam. Probably 25 years ago, the mechanic working next to me at a Chevy-Olds dealer had an Aurora with a misfire. It couldn't be felt, but it could be seen on the scanner. He replaced coil, plug and injector and it still missed. Compression was good. GM technical assistance told him to do a running compression test. The compression was low on that cylinder and it ended up being a bad cam. Until that time, I had never even heard of a running compression test.

 

Yes. I actually just edited my post.

 

And the reason is time... at cranking speed, there is sufficient time to fill the cylinder, even with a diminished valve lift. But when it's running, that time is not available, and lower compression pressure, from incomplete cylinder filling, will be apparent.

JW

 

I was just thinking about what the results may be and I don't recall what his readings were but could you get a higher reading if it were a bad exhaust lobe and a lower reading if it were an intake lobe? I mean higher and lower in relation to the other cylinders.

 

I was taught to read the number on the first compression stroke and continue cranking for three more compression strokes and read that number. The key is to compare to the other cylinders.

 

Thankfully I have not dealt with enough flat cams over the years to have a lot of experience in this area. One could reason that the readings might be higher with an exhaust lobe going away, if in fact your filling the cylinder more completely... but I think the reverse might be true, definitely in a high performance engine with headers... considering the effects of scavaging pulling fuel into the cylinder at overlap.

I think the readings will be highly dependent on what type of motor your working on, and the extent of the failure. I think variation from the norm is what your looking for, more so than high or low readings.

JW