Help. Runs fine then looses power

[drwhosc]

Ok I need help with trouble shooting. I have an issue I can not figure out. Just got the car back from an engin rebuild. The shop is 3 hrs away, and I just don't want to bring the car back there. So here is my story.

the car starts fine, runs well, plenty of power, and will pull all the way through the rpms. Then the car will miss and high rpms, and then progress to where it has no power. I thoought it was fuel related. I replaced the front pump, removed the in tank filter. So put everything back together, and the problem presists.

So now I have to chase down possible electrical issues. I have an MSD ignition with a pertronics ignitor It sure is behaving like bad points. So I am going to focus on the pertronics system. Any other trouble shooting tips befor i go an buy a new system. these things usually last, so I am skeptical. Also one more funny tidbit. If I shut the ignition off, and let things rest for about 3-5 mins, the car runs fine for a while.

thanks for you help

[edmayo]

Did it do this before the rebuild? Do you have all the same ignition parts that you had before the rebuild? What ignition coil do you have? Sounds like the coil is getting hot, then works fine once it cools down. Test the coil with an ohm meter across the primary winding when the problem occurs, if its Bosch refer to the workshop manual for the specs, if MSD ask them what it should be. Did you replace the spark plug wires as MSD suggests to the resistance type?

[Irish911]

Some other items to check, make sure your distributor is not loose and properly grounded. How soon after you start driving do you loose power? Next time it happens feel the CD box and your coil to see if either is hot. Test for spark when it is hot and has lost power. Don't wait for it to cool down before testing.

[drwhosc]

Thanks for the replies. Still a mystery to me. So in retrospect, I do think the problem was there prior to the rebuild. I was chasing a fuel issue and that is not it. I did rebuild a good chunck of the fuel delivery system. I still have not ruled out a vacuum forming in the gas tank because of poor ventilation. But everything else there has been addressed.

The symptoms are variable. the car can run for 40 mins one day, then 10 the next. Not really dependent on temperature either. I think I will probably start with looking at all the electrical connections. take everything apart and check all the connections. It really feels like a bad ground, or a short is developing.

Is there something on the electrical panel that I should check. I don't remember anything there other then the voltage regulator.

After that, I will probably replace the coil as it is the next likely culprit.

the weird thing about all of this, is that if I just turn the power off and restart, the car does fine for a while, so I am not sure it is totally temp related. This one is baffling, and I love intermittent electrical problems. At least the early 911s are simple electrical wise.

[ejboyd5]

Replace the coil.

[cmcfaul]

mfi or carbs?
What do the plugs look like?
any popping or leakage between the heads and cyl's when hot
How does the timing look like when cold and hot?
Any smoke?
Oil level not too hi when hot?
Did you increase compression on rebuild? If yes, are you running premium gas
Pressure fed chain tensioners?

Chris
73 911 E

[drwhosc]

WOW lots of questions..

MFI
Plugs look good, brand new, onlly about 40 miles driven thus far
Don;t know about popping between the heads. Had new Pistons and Cylinders installed How can I tell, I guess just listen
timing looks good. Really no difference when cold or hot. Can restart the car when hot no problem, will run fine, then symptoms return. Just have to turn off the ignition to reset system
Some smoke on initial start up, but nothing major. Clears quickly
Oil level does increase to 8 when hot, but still within specs. Have not checked with dip stick
compression at factory, and do run premium gas
Yes Hydraulic tensors.

Just bizarre behavior but just curopus wha tyou are after,

I guess rich vs lean running
No timing advance
increased crank case pressure and compression ratios
and valve clearance.

the engine will idle well at any temp. I have not let it run until it craps out. but starting will get difficult if I don't let it rest for a bit.

[Mr9146]

Are the fuel lines hooked up correctly from the tank to the pump, the pump to the filter console, and the filter console to the injection pump?

[edmayo]

Since the engine has been out are you SURE you got all the grounds hooked back up at the ground post,,,especially the ground strap from the relay board. Very easy to overlook simple connections.

[cmcfaul]

Could be MFI running too rich when warm as a lean condition would be very noticable (bucking and studdering). Try turning the linkage to the MFI a turn or two shorter to lean it out. If if does not help, very easy to undo.
The fuel line connectors from the filter to the MFI and fuel pump are pictured wrong in most all books and can cause strange problems. Double check they are correct. My first thought would be to the distributor. But a different coil and timing checked out OK should eliminate thatl

bad tensioner when hot?
maybe rings not sealing when hot? Alot of air flowing from the crank case breather tube?
MFI mixture?
Fuel line connections? Check as they are counter intuitive

Thats all I can think of

Chris
73 911 E