[DeTomaso] 302 diagnosis

[Will Kooiman]

This is a tough one to diagnose, but there are still only a few things it
can be.

1. fuel
2. spark
3. cylinder seal (rings and/or valves)
4. timing
5. anything else?

Since you've tried several carburetors, I'd bet it isn't the carburetor.  My
car was acting like that, though, and it turned out to be junk in the fuel
filter.  After the car ran for about 20 minutes, it would starve for gas and
die.  After it sat for a few minutes, the carburetor would fill back up with
gas, and it would run fine.  I thought it was temperature related, but it
wasn't.  It just happened to run out of fuel pressure about the same time it
warmed up.   Once I changed the filter, it ran great.

I've heard of similar stories about fuel lines with small holes on the
suction side.  It would suck just enough air through the hole(s) to pump air
instead of fuel.  If the holes are on the pressure side, you'd get a stream
of gas.  I haven't seen this first hand, but it makes sense.

It could also be vapor lock.

Of course, you can see any of those conditions with a fuel pressure gauge.
If you aren't getting 5-7psi at the carburetor, you've probably got a fuel
delivery problem.

I've heard of coils overheating and acting weird, but I haven't seen it
first hand.  You can test the resistance on the coil to see if it is good or
not.  You can replace it with a known good coil to see if there's a
difference.

If it were a valve not sealing, I'd expect one or two with low readings, not
all 8.  The rings might not be sealing, but it shouldn't get worse as the
engine warms up, should it?  A leak down test should tell you how much is
leaking and where it is leaking.  (as Jack D. suggested).

I don't see how it could be a cam or ignition timing problem if it runs
great when cold.  You could double check the cam timing with a degree wheel,
though.  It isn't easy with the engine in the car, but it can be done.  You
could also measure the valve lift for all 8 cylinders to make sure the cam
lobes are okay.  Measuring valve lift is pretty easy with a dial indicator.
If you want ballpark numbers, you can probably do it without a dial
indicator.

Good luck,

Will.

-----Original Message-----
From: detomaso-bounces at realbig.com [mailto:detomaso-bounces at realbig.com] On
Behalf Of dyogi
Sent: Sunday, February 24, 2008 4:03 PM
To: detomaso at realbig.com
Subject: [DeTomaso] 302 diagnosis

Thanks for all the replies!  

 

The cam and lifters are new and the break in seems to have gone well.  The
engine runs like a champ until it warms up although the vacuum always seems
low.  It runs great until it reaches normal running temperature, then the
idle starts to drop until it dies.  If throttle is applied, the engine will
continue to run well but not idle.  It will restart easily after it dies.  

 

I had the motor out because I was having problems with the engine dying
during drives.  I had thought it was the timing chain and replaced that as
the old one was worn and stretched.  While the engine was out, I replaced
all of the gaskets including the head gaskets.  The bores looked in really
good shape with almost no ridge.  The heads also looked good so I only
changed the valve seals.  The engine has 24,000 original miles on it.  No
exhaust popping or backfiring.  The motor runs really good until it is
warmed up.  Starts right up and idles great even when cold.  I guess that
the timing chain was a factor in the original problem as the engine now runs
great until warmed up.  It barely started when I first pulled it out.

 

Thanks all!

 

Dennis

 


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