diagnostic dillema

[krines]

Just had a top end overhaul and now the plane is acting up. On maiden flight noticed sluggish power and popping on take off roll. Sounded fine on runup, mag checks in limits. Later noticed the engine rough at idle and any power setting. Switching to L or R mag made no difference. EGT and CHT shows cylinder 2 absolutely cold. Plugs pulled and checked out OK and producing spark. Wet gas noted on plugs. Wires and timing check out OK. Plugs all replaced and engine run up, the engine is still rough. EGT and CHT show cylinders 4 and 6 runnning cooler than all odd number cylinders, and cylinder 2 cold.
Compression checks OK and valve covers removed showing all is well. Any ideas from you rocket scientist out there. Currently the mags are being pulled and checked but this does not seem to be the logical cause.

[johneeb]

Really bad Primer leak??????????

[krines]

Thats a great idea as I have suspected a primer leak in the past totally unrelated to this issue. Good now I have something else to do after work. I will let you know if you are a true rocket or should I say 170 scientist

[Jimmy M.]

Would the pushrods have been mixed up at overhaul ??
If some of them are too long they would probably show good compression but when the lifters pump up with the
engine running they would keep the valves open..................just a thought

[Brad Brady]

I'm with Jimmy....sounds like a valve issue,or lifter?.....Brad

[jrenwick]

Verify that the spark plug wires go to the right places?

John

[Bruce Fenstermacher]

First is the EGT/CHT wired to the correct cylinders. I just recently saw this where there was a rough engine and the mechanic went after the cylinder indicated by the instrument. Didn't find anything. Pulled a valve cover from another cylinder for the heck of it and found a bent pushrod. Found the EGT/CHT wired wrong by a pro shop.

I would confirm all is together right. Then I'd pull the valve covers of all cylinders and watch all the valves open and close.

It sounds to me like you have a valve stuck in the open position. Did you check all the compressions?

Start the engine and if ruff run it about 30 seconds and shut it down and them feel each cylinder. The cold one is your dead one.

[Brad Brady]

Verify that the spark plug wires go to the right places?

John

Good idea John, I was assuming new top...new leads (that's the way I do it)....if not, check the continuity to the cly.s....Having both bad is remote...but not out of the realm of possibility.......Brad

[DWood]

Sometimes intake leaks will cause popping such as the primer not connected to intake manifold???

[Brad Brady]

Sometimes intake leaks will cause popping such as the primer not connected to intake manifold???

Yea, I agree...but an intake leak should show a high EGT.....You know...running lean.....Brad

[DWood]

EGT and CHT shows cylinder 2 absolutely cold. Plugs pulled and checked out OK and producing spark. Wet gas noted on plugs. Wires and timing check out OK. Plugs all replaced and engine run up, the engine is still rough. EGT and CHT show cylinders 4 and 6 runnning cooler than all odd number cylinders, and cylinder 2 cold.

Out of curiousity, when you did the compression check on Number 2 were the valves opening as you moved the prop? If so, the plugs are wet on number 2, it is probably the spark plug leads on wrong plugs.

[cessna170bdriver]

Yea, I agree...but an intake leak should show a high EGT.....You know...running lean.....Brad

Not necessarily. If the leak is severe enough, the cylinder will be cold. Been there, done that. Forgot to tighten the hose clamps on a manifold to elbow connection.

Push rod mix up was the first thing that occurred to me. Deflate lifters and check valve lash. Symptoms are also like a stuck valve but not likely that soon after a top. Still worth a check.

I'd think swapped plug wires would have shown up on the first run-up. Still worth a check though.

Miles

[blueldr]

Miles,

How do you deflate the lifters?

[krines]

Ok guys the rocket (cessna 170) scientist award goes to DWood. Last night I removed the baffles and lo and behold the number 2 cylinder intake manifold gasket was sticking out the side. So replaced the gasket and its fixed. Intake leak is the cause. Some notes of interest. That number 2 cylinder was actually stone cold to the touch after a 5 minute runup. The 4 and 6 CHTs were running 50 to 100 degrees cooler than the odd number cylinders. Was that because 2 was too lean to fire and then the leak allowed all the intake fuel to go to cylinders 4 and 6 causing a richer mix and cooler temps? Spraying ether around the intakes failed to make the diagnosis. I also failed to mention that after the top my idle speed had increased from 650 to 1000rpm. All is well with the world once again I'm no longer grounded. Thanks for all the incite. It is fantastic to have this forum around for all the help one can get very quickly. N1320D away

[GAHorn]

Keep in mind that any intake leak at the #2 cylinder "elbow"...is directly connected to the intake "spyder" down near the sump, so will allow an overly lean condition to exist for all cylinders on that side.