Might be a blown head gasket, might not... How to tell?

My '82 Mazda 626 - 2.0 liter inline four with no computer, in front of a
5 speed stick - has been giving me some weirdness lately that I simply
haven't been able to track down. It's intermittent, so that's not
helping the troubleshooting process. I *THINK* I've got it narrowed down
to two "mostly likely suspect" culprits: Fuel/carb issues, or blown head
gasket. Trouble is, the symptoms could be accounted for by either, or
might even have nothing to do with either one.

Basically, anytime I "get on it" a bit (Climbing a slope and put the
hammer down a little extra to maintain speed, "stomping" on it -
figuratively speaking - while pulling out for a pass, or substantial
speed alteration while cruising) I might get one or more "hiccups"
before the engine really "kicks in" - as if perhaps the extra pedal is
riching out one or two cylinders for a moment. This misbehavior is
SOMETIMES accompanied by a few quiet "pop-pop-pop" type backfires out
the tailpipe, other times, it's not. On a might-be-related note,
occasionally, I get a single, noticable (but not particularly
impressive) "pop" out the tailpipe at shutdown, usually about 3-8
seconds after the engine has stopped spinning - The timing of this
"shutdown pop" is invariably such that if I turn off the key and
immediately start to get out of the car, the "pop" will happen just
about the time I'm flipping the door shut. This "popping", whether it
happens while asking the engine to work harder, or at shutdown, seems to
say to me that there's too much fuel getting dumped into the beast for a
complete burn inside the cylinder(s), so the leftover is burning in the
exhaust.

Generally, dropping RPMS below about 3K will "clear" the problem when it
happens, but there have been a few times when I've darn near come to a
stop before getting the engine to "even out" and pick up the load.

Redline on the tach for this car is 6K, with the "sweet spot" running
around 2800-3200 RPM depending on gear. book specs max output of this
engine to be at 2750 - my usual driving and shifting habits tend to keep
the revs between 2500 and 3000 unless I'm doing sustained high-speed
work like freeway travel, climbing a slope, or substantial speed
alteration. Freeway driving on a 70 speed-limit stretch, ferinstance,
usually puts the revs in the 3300-3600 range in 5th. Hill-climbing might
have me winding it up to the same range in third to keep to the
double-nickel - Can't use 4th/2500-ish for that, as doing so will lug
the engine bigtime when trying to climb. Likewise, dropping down to
second isn't a practical option, since doing that would have me trying
to wind it out to about 4K to keep up with traffic on the slope.

All in all, when this happens, it feels a LOT like back when I had
timing trouble due to the distributor "lockdown" bolt being loose,
allowing the distributor to flop around and throwing timing all over the
place at random. That isn't the issue this time... That was the VERY
first thing I checked when I started seeing the problem. Timing is as
close to dead on as I know how to get at 5 BTDC at idle with vac.
advance disonnected/plugged, exactly as specced, and the distributor is
solidly bolted down.

I've considered plugs/wires/rotor/cap as the culprit. Replaced 'em all.
The car was due for a new set anyway, although not desperately. No
change.

I've wondered about a plugged exhaust - That doesn't seem likely,
though, unless perhaps there's a chunk of something that shifts around
to plug it up, then falls away to free the exhaust path. This problem
doesn't happen either predictably, or consistently. There's a much
better chance of me seeing it happen in certain situations, but
reproducing those situations *DOES NOT* always reproduce the problem - I
may have it hit while climbing what's known around here as Lakeside Hill
6 nights in a row, then not see it happen at all for several nights,
then have it happen once, and not happen again for a week. The
intermittent part is what's driving me nuts.

At this point, I'm wavering between suspecting fuel issues and a blown
head gasket. (And if that turns out to be the case, I'd bet a dollar to
a donut that it's blown between the #3 and #4 cylinders)

As far as fuel, I was suspecting the float may be acting up - I
sometimes get a similar power drop-out while under acceleration on the
flat and the road is curving to the right - Nowhere NEAR as severe as
when I see/hear/feel it when asking the engine to "work harder", but
enough to be noticable as a distinct "miss" or "drop-out". Thing is, I
rebuilt this carb *FOUR COMPLETE TIMES* just a bit over a year and a
half ago, each time even more carefully than the last, while trying to
track down why the beast was failing California smog. (Mondo-high HC,
low CO2) That problem turned out to be a cracked vacuum hose controlling
the air-injection valve. As I said, four complete carb rebuilds, and
each one done more carefully than the previous - I wouldn't be surprised
to find out the carb was/is in "better than brand new" shape when I
finished that little fiasco, so I've got some serious doubts as to carb
problems. I can't rule them out completely, but it just doesn't quite
fit with what I know.

The head gasket is likewise a suspect. But not seeing the problem
consistently makes me wonder if it's even a reasonable suspicion.

I've got no idea how many actual miles on this engine - Speedo cable was
broke when I bought the car it was originally in, had been for an
unknown amount of time, and stayed broke for the better part of a year
as I drove it around a high-milage paper route and worked on other more
important issues with the car. Now that I've swapped the engine into
this vehicle due to the wreck that wiped out the original one, I'm even
more clueless as to the actual miles the engine has gone. Supposedly,
this engine was rebuilt (for the original vehicle it was in) back in
'92, but I have no way to confirm or refute that claim.

It uses a little oil - 1/2-3/4 quart or so between oil changes, with, I
suspect, most of that being burned after bypassing the valve-stem seals
- Some mornings, I get a puff of blue smoke at startup and for a short
time after, and occasionally get some smoke on *REALLY* heavy
acceleration, but otherwise, nothing noticable in that department.

All four plugs were scaled heavily, #1, #2, and #3 with a gray-ish tan
"compressed powder" that flaked right off easily with a toothpick when I
did the plugs/wires/cap/rotor change. Once I get rid of that build-up,
the electrodes on the plugs looked close enough to brand new. The #4
cylinder's plug was *VERY SLIGHTLY* moist - just barely enough to detect
- and a good bit darker in color than the other three, suggesting to me
that it's probably the place where the oil is going. The rest were dry.
Like the others, the #4 plug cleaned up loking practically new.

Coolant in the radiator has a very slight rust/brownish tinge to the
usual green color, and when I let it cool and pop the cap, sometimes
shows a "froth", like the head on a glass of beer. No bubbling that I
can see when looking in the radiator with the engine running. No sign of
oil that I can find. Drawing a coolant sample while the engine is
running, putting it someplace to "settle", and checking on it later
shows only the slight brownish tinge I mentioned, with no oily layer
floating, or anything settling out - basically, looks like healthy
enough coolant that's been in contact with a little rust.

No water in the crankcase that I can tell. Oil filler cap usually (and
always has, since the day I bought the car) shows a TINY bit of
"mayonaisse" in the deepest crevices, but nothing more than I'd expect
from condensation. Definitely nothing even remotely like what I'd expect
to find if there was any significant amount of coolant leaking into the
oil.

Usual head-gasket failure on these engines is the "skinny spot" betwwen
the #3 and #4 cylinders, sometimes between #1 and #2. Never heard of one
that broke between #2 and #3 without filling the crankcase with
water/radiator with oil.

So, WITHOUT lifting the head, is there any practical way to confirm/rule
out a blown head gasket? I'm stumped on this one.

I guess I *COULD* "shotgun" the problem by swapping in the freshly
rebuilt spare engine I've got sitting here, but that seems like an awful
lot of effort when the current engine is running QUITE well - MOST OF
THE TIME...

Anybody got any thoughts for me?

--
Don Bruder - - New Email policy in effect as of Feb. 21, 2004.
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