78 280z stock engine mystery please help!!!

[Jeff 78 280z]

Hey everyone I bought a 78 280z 5spd a few months ago. When I purchased the car I drove it around the block and stuff. At that time I noticed that when I pressed the gas the engine would pop and sputter. After I took my foot off the gas the engine reved I could press the gas again and the car was driveable. I tried to drive it the 100 miles home but I got about 15 miles down the highway and the engine slowly lost power and died. The fuel filter seemed clogged but I hate diagnosing on the side of the highway.

 

I got the car towed to my home and went to work on the engine. This is what I have done to date. First thing I did was change the Fuel filter and the engine started and ran just like it did before it died on the highway.

 

New rebuilt injectors

new fuel pump

new fuel pressure regulator

new spark plugs

new cap

new spark plug wires

cleaned all electrical connectors with deoxit

replaced a few corroded connectors Going to the Thermo time switch and Water temp sensor, Auxillary Air Regulator. Bullet connectors too.

Blew out all the fuel lines

Replaced fuel filter

replaced all vacuum lines

discovered the diaphram on the carbon canister was leaking. It now is reseated and sealing fine.

Set the timeing to 10 degrees before TDC. Previous owner had it set so advanced it wasn't even on the degree marker.

Replaced the intake/exhaust gasket cause the old one was leaking.

Redid the wireing to the fuel pump from behind the passenger seat back.

Cleaned all the grounds I could find in the engine compartment.

Did all the tests in the FSM at the computer connector. All seems ok

Installed a Fuel pressure gauge after the fuel filter. At idle the pressure is about 30psi and rises when I hit the gas. With the fuel pressure regulator unplugged the fuel psi is just above 40psi.

installed a new gasket on the oil filler cap

 

Ok now that that is out of the way. I hope someone can help me. I used a vacuum test gauge and when the engine idles it is at 18psi and steady.

 

Now for the problem.. OK after I changed the timeing to 10 degrees btdc the car would only idle. If I touched the gas pedal at all it would stall. So being curious I removed the cover on the AFM and turned the counter weight by hand. When I did that I could rev the engine and everything seemed fine. Obviously I can't hold and turn the counterweight and drive my car. I tested the AFM everyway the fsm says too. And being curious I disconnected the AFM and to my surprise the engine would rev and I could drive it but it would run way too rich. It would seem it was running too lean with the AFM plugged in. I tried to loosen the coil spring in the AFM by turning the Big gear to make it run richer. I did it a few teeth at a time. But it got to the point where the spring was so loose it wasn't practical So I bought a new Bosch AFM. I installed it yesterday and now the car won't even run.

I hit the key and the engine sounds like it wants to start but no dice.

 

This problem is a mystery and I am about out of ideas. I thought for the longest time I had a vacuum leak but after the 18psi steady vacuum test and the fact that the engine will actually rev and drive without the AFM I am inclined to think otherwise. Its really strange. With the old afm the car would idle well but die when I touched the gas. Disconnect the AFM and I can drive the car and rev the engine but it surges at idle until it eventually stalls if I don't blip the gas pedal. With the new AFM it won't idle at all.

 

I have a spare computer and I will swap it out tomorow to see if there is a change. Oh yeah I tested the Thottle Posistion Sensor and that tests fine. This engine problem is a pain. I can't seem to figure out if its a vacuum leak or an EFI problem. It seems the cause is being masked by a number of symptoms that all together don't make much sense.

 

I have put alot of time into this car. This is also my 4th Z. I have built and installed a 383 sbc in a 75 280z that went smoother than this. I really would appreciate some help!!!

[hoov100]

Set the motor to TDC on the compression stroke on cylinder 1 and make sure the dizzy is on the correct tooth. Once you have confirmed that, set the timing to 15* and make sure the TPS is on the idle contacts at idle and not hitting the WOT contacts until it's WOT.

[Larry Mahanor]

Hi Jeff:

 

I see you live in Key west and I'm trying to figure out if we've met, as I'm from Ft. Lauderdale and run the Bay Bottom Crawl evey year at Sugarloaf Key in my Z. There was common problem in the 280Zs associated with coolant temp sensor, which is mounted in the thermostat housing. First off, take a look at spark plugs after you experience the problem. If they are very fouled with black sooty deposits, then the engine is running so rich that it can fire the plugs. If so the take a look at the two wires that connect to the sensor. There two round "bullet" connectors on top of the intake manifold, parallel with the throttle shaft. They typically get corroded and make poor contact. Also check the electrical connector that attaches to the coolant sensor, and don't rule out a defective sensor.

[JSM]

Funny, I haven't dove into my 78 but your inital symptoms are exactly the same. Starts right up idles great go to drive it and lots of poping, no power, etc.

 

Interested on what you find out.

 

Maybe it is a Jeff, Florida thing. As we both are Jeffs and both live in Florida.

Edited June 25, 2010 by JSM

[Jeff 78 280z]

Ok I switched out the computer with one I bought off of ebay. The engine started for one second then died. So I unplugged the afm again to see if the engine would run with a different computer and the afm uplugged and nope nothing. So I guess I will try what hoov100 has suggested. Now that you mention it that would make sense. That the timing would be off. I hope that its something like that rather than a short in the EFI harness. Although like I said I have done all the tests in the FSM from the computer harness and everything checks out ok.

 

 

Hey! Larry Mahanor I'm pretty sure that we haven't met I have never done the sugarloaf run. I used to be part of a sports car club here is Key West back in 1996. Maybe if I can get this Z running right I may join you next time. I have checked out the bullet connectors and I have already replaced from the bullet connectors to the thermotime switch and water temp sensor. I tested the sensors and they seem fine but were a little corroded. I should probably replace them.

 

Thanks I will do what hoov100 said and see if that was the problem probably on monday I will get back to tell you guys what happened.

 

 

Thanks again!!! Wish me luck!!!

[greghassen]

"Previous owner had it set so advanced it wasn't even on the degree marker."

 

 

 

Sounds like your timing needs more advance. Your harmonic balancer may have slipped

its outer pulley. I'de try timing it by "ear" and see if the popping goes away.

 

Also sounds like you still have a vacuum leak. shouldn't a 6 cyl have 19-21 inches of vacuum?

when you take the oil cap off while its running what does it do?

[Tony D]

Check TDC before making timing adjustments! It moves. This wouldn't be the first time someone messed up a properly set car by resetting the timing on a slipped pulley.

 

If it ran 15 miles, likely it's not the ECU. Too bad about the ECU bought. Check the links on the EFI Bible and the FSM for your year, the troubleshooting and explanations in those two areas really clear up the 'mystery box'...

 

I'd do a basic set of checks from the FSM troubleshooting section, including testing the advance on the distributor (both centrifugal and vacuum).

 

And I'd check if the injector connectors have 'the green grunge' on them, cleaning them usually helps with lean running.

[Jeff 78 280z]

Thanks Tony D. The injector connectors are new I'll check the fsm for the trouble shooting on the distributor. I didn't think the timeing was off after I set it cause it runs and drives with the AFM unplugged. I will try timeing it by ear first if that doesn't work then troubleshoot the distributor as per the fsm.

[Tony D]

" I will try timeing it by ear first if that doesn't work then troubleshoot the distributor as per the fsm."

 

Enjoy your process. When you want to do things scientifically and actually solve the problem with the least amount of drama, let us know.

 

Good Luck!