'92 Plymouth Voyager 2.5 4cyl top-end noise / pre-ignition?

I recently completed an engine rebuild on a 2.5 4cylinder engine in the
above vehicle. I did the rebuild because the engine was burning about a
quart every 40 miles and had no oil pressure. Also, it had a lot of what
sounded like lifter noise.. a real racket, it drowned out the radio at
highway speeds.. Which I attributed to the lack of oil pressure. This is
an overhead cam engine.

After the rebuild, the engine has great oil pressure.. lots of power,
everthing is great. Except, there is still significant top end noise
when idling and a lot of what sounds like pre-ignition while driving. It
is possible that what I am thinking is top-end noise is actually
pre-ignition when idling. The vehicle is timed (12degrees) and runs like
a champ.. It just sounds like a diesel. The knocking disappears when
accelerating, and becomes very pronounced when decelerating.

At first, I thought that perhaps the oil-channels in the block were not
allowing significant oil pressure to the top end.. Blocked, maybe. But I
don't see why the oil pressure should change on deceleration or
acceleration, the crankshaft is still rotating at roughly the same
speed, so should be about the same oil pressure... I'm settling on this
being an ignition problem, but there aren't that many ignition parts in
the vehicle. Does anyone have any thoughts?

Thanks,
-Adam

 

Piston pin knock. The 2.2 and 2.5 litre engines with original pistons are
prone to it. The engine carries on running normally, but eventually it
gets so loud that the engine does sound like a diesel. If you'd fix it,
install new pistons.

DS

 

I see. Should I worry about tearing it back down again? I never thought
to replace the pistons. I did the rod bearings & rings and put a brand
new head & cam on it.. One of the original piston rods was pretty bent,
so I replaced it with a piston from a yard. Will anything bad happen?
(Aside from loss of hearing, or the purchase of louder and louder
stereos)...

 

Well...that sort of depends on your priorities. As the piston pin bores
get more and more wallowed out, the pounding effect becomes greater and
greater and the wear accelerates. Eventually you'll break something,
probably a piston, and then bad stuff'll happen (e.g. rod through the
block), and then all your rebuild work will go "ker-flush". Also, the
noise will keep getting louder and louder, which will not only remind you
every time you drive the van that something's wrong, but will also make
the van impossible to sell, if you were ever inclined to do so.

 

Oh. How many miles / months do I have? I don't think I can afford to
tear it back down inside the next 6 months.

 

I don't think your engine will grenade inside the next six months, but
take it easy.

 

Well, thanks for all of your very knowledgeable help. I'm glad to know
what it is anyways. I wish I'd correctly identified that particular
noise originally, then I wouldn't be tearing it back down again. Is it
possible that this is the reason that one of my rods was pretty
seriously bent? Or was that due to something else? (like low oil pressure).
Why would I not hear the wrist pin knock on acceleration? It seems like
they would knock more at the higher revs.

 

When you say rebuild what exactly did the rebuild consist of? If you suspect
it to be a wrist pin noise or lower end bearing noise take each plug wire
off one at a time to see if the noise goes away. take one off put it back on
and take another one off until all 4 were isolated.

Glenn Beasley
Chrysler Tech

 

Thanks, I'll have a look.
I replaced the head with a new cast, including valves/springs/cam etc..
rings, rod bearings. I didn't pull the engine, so the main bearings
didn't get done.. and the cylinder ridge didn't look bad, so I honed
them and reinstalled the original pistons. One of the connecting rods
was bent, so that got replaced, along with the intermediate shaft (which
was missing a couple teeth) and the oil pump & screen.

 

It looks to me that what I'm describing is Piston slap. I noticed some
odd markings on the sides of the cylinders when I pulled the pistons..
But as the esteemed Nomen Nescio points out, I'm an idiot, and didn't
think of the implications.. I just honed it and tossed in the old
pistons. What causes this? If I bore the cylinders out and put in
pistons to match (along with the main bearings that I neglected) will I
be in the clear? It seems that in the end, this will cost what a mexican
rebuild would have.

 

Ring ridge is a thing of the past, if you notice I bet you still see the
cylinder hone marks in each cylinder. What you are probably hearing is
wrist pin noise. That was a common problem on that engine.

Glenn Beasley
Chrysler Tech

 

Pretty sure it's Piston slap. I checked the discarded piston&rod and
there was no noticeable play in the wrist pin. However, after about 3
days of driving there is plenty of fine metal dust in the oil pan and
the hone marks are worn on each side of the cylinders. Also, the piston
that was replaced is worn smooth on the sides as well. The really odd
thing (to me.. I know very little about these engines) is that the top
of the cylinder was round and I have very good compression in each
engine. I read something that said that these engines are basically
stroked 2.2's and the piston skirt extends beyond the bottom of the
cylinder well.. So I suppose the bottom of the cylinder could be ovalled
enough to cause slap while still having good compression.

 

Just ignore No-Man - if you bothered to google some of his past posts
you will see he's a friggin ijiot.

Ted