For a while the engine temperature has been too high. I've replaced the
temperature sending unit with no change. Now I've noticed that there is
always air in the system. It doesn't matter how many times I try to
bleed it, the air always comes back. Maybe the the temperature sender
measures air temperature that is higher than the water temperature?
Where are the most probable places that air can enter the system? I
guess it should be on the low pressure side of the pump. Is it possible
that the pump itself is faulty? It is only 6 months old.
I know that there is something wrong with cylinder no4. The spark plug
is black (not oily) and there is lower compression at that cylinder
(12.5 bar, the other 14- 14.5 bar). The hot water leaves the engine
close to cylinder 4. Is it possible that the faulty cylinder creates
extra heat that will cause the higher temperature?
It might be possible to re-torque the head to correct a slow leak in the
head gasket. Of course replacing would be the best and isn't too
difficult on these engines.
'91 100Q 5spd
As others have noted, it possibly is a head gasket leak. My '87 5ktq had
one, and a few other issues. You may find this helpful:
- on the head gasket issue, if any plug has a whitish deposit on it, then
the coolant is leaking into the cylinder.
- the leak may be intermittent - it may leak more when the car is cold, or
- Oil in the coolant (brown/blackish gunk in the reservoir), coolant in the
oil (milky residue on dipstick) may signal head gasket leak.
- try this: run the car up to operating temp, then pull the plugs and hold
an inspection mirror over the plug holes -> fog = coolant leak in that
- pressure testing the coolant system may show quick oscillations,
indicating cylinder pressure in the coolant system
- flared radiator connections may be a sign of engine pressure in the
cooling system (or an overzealous mechanic tightening the hose
My '87 was always running hot - slightly under 100C most times, etc. I had
the head gasket replaced (leaking in #5), flushed the coolant system and rad
a number of times, nothing seemed to work. I bought a nearly new Modine
metal rad from the local scrapyard (it was cheap, I figured I needed a
spare) and wouldn't ya know it, the plastic OE rad blew up (bigtime!) I put
in the Modine and the car runs nearly 40C cooler on the highway - only
venures into rad fan territory in stop 'n' go traffic. The old rad sat in
my garage for a while - I cut it up and, lo and behold, the gasket
separating the coolant in/coolant out portions of the end tank was broken -
ahHA! The coolant was bypassing the radiator fins.
Important point to note: If you are contemplating getting the head rebuilt,
make sure they use new head bolts, not just the old ones (ask for the spare
parts in a box). The head is attached with bolts that will stretch when
properly torqued. If reused, the stretch bolts will bottom out in the
threaded holes in the block and you won't get the right clamping force on
the head gasket - leading to another blown gasket. That is what the
mechanic found who replaced the gasket on my car - the bolts had their ends
ground down to avoid bottoming out. They still did on a couple and poof
went the gasket. If the car has been repaired in the past like mine was,
just retorquing the bolts will not help (they're bottomed out and won't
tighten any more).
1987 Audi 5kTQ - rejoining the realm of happy cars after being seriously
abused by PO's
1980 Audi 5k - always pampered
1962 and '64 Auto Union DKW Junior deLuxes - abused by time.
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