Ruff Running Nova

[k9hotrodder409]

Hi guys, I have a problem with my grocery getter.It's a 1973 Chevy Nova with a 250 ci six and a glide. When the motor is first started up it runs fine. When the motor gets to normal operating temp it runs like sh**.It seems to have a very bad miss. A complete tuneup was in order and was done.No change in the engine. Still runs crappy when up to operating temp. Replaced carb with a rebuilt from National Carb in Florida. No change.Took a compression check when engine was cold. All cylinders around 150psi and no bleeddown.Did notice when engine starts to run rough the tail pipe makes a sound like a steam engine. Bad Valve or guide or cracked valve seat in the head ? The fast idle solenoid was not working and a N.O.S. one was ordered on e-bay. I am waiting for it to arrive..The puffing sound from the tail pipe has me worried as the car has only 68,000 original miles. I bought the car from the original owner. I thought these 6's were bulletproof. I guess not. Any ideas ? I'd appreciate any help.: Thanks Butch

 

[Ronnie Russell]

Spark plugs should tell you if any cyls are getting water. I would suspect a cracked head.

 

[oil4kids]

headgasket, or someone forgot to put anitfreeze in it one year, had a 74 nova 250 6cyl the engine is nearly indistructable

the motor was designed by non other then Dick Keinath aka Myster motor engineer

 

[WENGINE]

Butch did you try the vacume advance Didn't make a difference. Ken

 

[k9hotrodder409]

Thanks Ronnie ,Mark & Kenny. The plugs were all the same color when I put the second set in last week.No indication of water in any of the cylinders and the rad shows no coolant loss. The vacuum advance is new as of 2 weeks ago,and seems to be working fine. I am going to try another compression check with the motor at operating temp and see what that says. Thanks again guys, I will keep you posted.

 

[bigblockwilly]

could be a clogged EGR valve but i dont know if yours is equipped sounds like somthing plugged in the exhaust if there is no obvous cooling system problems, just a sugestion

 

[62impala409]

This has got to be either a bad condenser or bad coil. I have had a condenser go bad and get a bad misfire. When my coil went bad it would only run crappy after the coil got heat soaked. This all happened many years ago but I still recall the details. Good Lock. Leo

 

[k9hotrodder409]

Thanks for your input Willy & Leo. The EGR valve is not connected it was disconnected long before the symptoms arrived. The complete pollution system was removed 2 years ago.The cars mileage increased dramatically after that. The diaghram moves up and down with my fingers.The Coil was changed with one from a 1971 Chev 350 engine ( In good working order ) The distributor condenser was changed with a new piece with the tuneup. I have seen these go bad in a hurry so I will change it this weekend with another new piece.: Thanks to all you guys. I will keep you posted with my findings. I know that I am looking right at the problem but for the life of me I can't figure it out. I have been getting a lot of brain farts lately.

 

[LongIsland63SS409]

I am going to try another compression check with the motor at operating temp and see what that says. Thanks again guys, I will keep you posted.

Butch:

A leakdown test would be better since you would hear hissing from the exhaust/intake or bubbles from the radiator.

Mike

 

[09Jud]

Is the intake manifold part of the cylinder head? those were the worst 6 cyl. chevy ever made. If I remember they cracked all the time and the crack opened up as the engine warmed up.

 

[OldTruckNut]

As was just mentioned, if it has the integrated head they were very prone to internal cracks. If it is the old style with the bolt-on intake, be sure to check the intake/exaust manifold bolts. I have seen them get loose and make the engine run bad when hot. Best of luck.

 

[k9hotrodder409]

Hey Old truck, This motor has the separate intake exhaust manifolds. I was thinking along those lines also but when I sprayed the gaskets with starter fluid I got no difference in the idle. I'm thinking the gaskets are not blown.I see not too many things left to check on this one .My wife is Pi****. This is her baby and I am running out of band-aids.

 

[oldskydog]

Crank it up at night in a dark garage and see if you get a light show. Distributor arcing, crossfiring wires, etc.

 

[64ss409]

Did notice when engine starts to run rough the tail pipe makes a sound like a steam engine.

I am wondering if you might have a broken valve spring With the engine running, have you pulled and replaced spark plug wires one at a time to isolate a cylinder?

 

[k9hotrodder409]

Thanks for the info Ron. That is just one more thing to add to the to do list this weekend. I will post my findings on Monday I hope. Thanks again to everyone for all the input.