1977 280z: Randomly running VERY rough

[ckurtz2]

Hey Guys,

       Back again, always with a broken car 😕 Anyways, I have been redoing my interior over the past six months with dash/ dash harness/ gauges etc all pulled. Over this time period, I have been starting and warming up the car under the pretense that the entire EFI harness is separate from all that stuff. It has started and idled well countless times during this several-month process.

Anyways, I got it together yesterday enough to get a seat in and go for a nice cruise around the neighborhood (still with no dash/gauges etc) to get the diff oiled and such. It ran perfect, no hesitations, good idle/part throttle/ WOT etc. Pulled it into the garage and left it there overnight. Started it this morning, because I was all of a sudden very proud of the car again after all the interior work. Anyways it was almost slow to turn over, but then the starter sped up and sounded like normal. Kind of started like a locamotive, puff puff puff then died. It will stay running under part throttle, but sounds like a 2 stroke running on 1 cylinder.

Anyways, typical. Started by checking if it had gas, put in 5 more gallons just to make sure (don't have gauge). Checked fuel pump, runs good under start and "on" when I open the AFM vane. FPR works good holding constant pressure. pulled the supply wire off the dizzy cap and put it next to ground. Turned over engine and saw the spark zapping so I know I have decent spark (looked really good). Pulled out the ole EFI Bible and popped the ECM module harness off. Did the full diagnostics, every ground checked in speck, AFM marked good at the harness, CTS read proper Ohm for the ambient temp, throttle sensor good. Did voltage tests, all good as per EFI Bible. 

Ok well just to make sure, checked voltage at injectors, all good which means dropping resistors are good. Both pins on each of the injector clips read +12.6 volts with one clip connected at harness and one at chassis ground. not sure if both should read voltage or not, but they did.

Ok started it and held throttle open. Pulled injector plugs at each cylinder 1 at a time. Affected RPMs on all cylinders, so all my cylinders firing just like crap.

Ok, so I decided to peek mechanical timing, popped valve cover off aligned at TDC and cam sprocket looked good with notch in proper place as per FSM and had the nice dog ear cam lobes. Dizzy rotor looked also like it was in right place.

Sweet, so I am getting spark, getting some sort of fuel, definitely air, etc. Pulled plugs and slightly wet and freshly black which means running rich. Interestingly enough, no black smoke or other smoke out the tailpipe though.

Any ideas of what could be going on here? I am not running a cold start valve, Thermo time switch, or BCDD. It is definitely very weird that I parked it and over night it has gotten this bad. I think some sort of contact got hot and then disconnected when cooled. Not sure.  

As always, thank you

[Zed Head]

41 minutes ago, ckurtz2 said:

good idle/part throttle/ WOT etc. ... Started it this morning, ...was almost slow to turn over, but then the starter sped up and sounded like normal. Kind of started like a locamotive, puff puff puff then died. It will stay running under part throttle, but sounds like a 2 stroke running on 1 cylinder.

Hate to say it but the WOT then problems in the morning kind of hints toward a head gasket problem.  Like you got some coolant in some cylinders, overnight.  I'd pull some plugs and see how they look.

[ckurtz2]

9 minutes ago, Zed Head said:

Hate to say it but the WOT then problems in the morning kind of hints toward a head gasket problem.  Like you got some coolant in some cylinders, overnight.  I'd pull some plugs and see how they look.

Could be, hope not. Wouldn't this cause white smoke out the tailpipe? I would admit the exhaust smells very odd from the usual, but its running so bad I am not alarmed by it. What would the plugs look like? I pulled 1 and 4 earlier today and 1 was wet and black and 4 was black and damp. This is after attempting to diagnose all day with lots of poor running and cranking, so I would think the wet is gas. 

[Zed Head]

Aren't wet and black and black and damp the same thing?

If it's running super rich check the FPR vacuum hose for fuel.  Maybe the FPR diaphragm failed.

[ckurtz2]

@Zed Head good point, 1 was very wet with fluid all over (smelt like poorly burnt gas) and 4 had little dots of moist stuff on them, but weren't soaked. I think I will do a compression test this week to see if a head gasket is possible. No mixing of oil or coolant, so only failure would be from cylinder to atmosphere or between cylinders. Oops my fault, forgot to mention I am running the Pallnet fuel rail with an aftermarket FPR. I can check to see if anything is leaking through the vacuum line.

[ckurtz2]

You got me onto the right track @Zed Head. Sad to say it is a bad day, a very bad day. Anyways, pulled #1 plug again and really paid attention. I determined the fluid was oil and not gas. The head gasket looked possible. Okay, compared the smell to the dipstick, and man it matched, so the fluid was indeed not gas. Did a compression check and shows 0psi with the throttle fully depressed. Really was thinking head gasket at this point. All other cylinders showed close to 160psi. Should have just done a compression check first, but the motor has like 300 miles or less on it since a head rebuild, and usually its the Bosche system giving me hell. 

I pulled the valve cover again and performed a closer look. To me, it looks like the intake valve on #1 is stuck slightly open. I have a photo showing the spring looking like it is not all the way up when both cam lobes aren't touching.. I Wish I had a leak down tester to properly diagnose. I am thinking that the valve is bent. I would assume this would be from valve float, but I really did not push the engine hard yesterday, and that would cause immediate damage, not appear once parked. 

Now, the car was going to be sent to a shop for a ticking noise I had been hearing on startup for half a year. It was kind of loud, happened when cold (however I did hear it a few times with warm engine), and only happened on startup for about 2 to 3 seconds. I figured it was just the engine building oil pressure, or maybe timing chain slap. Either way I was going to have a shop check it out to make sure. I noticed no issues with the noise besides the sound at startup. I still can't confirm that this noise was #1 or a separate issue, but I am betting they were related.

Now if it was the valve wouldn't that bend or get stuck while driving and then the engine would run like crap? I find it super odd that the car pulled into the driveway perfectly, sounded great. Then the next morning it has 0 compression. Maybe when the car cooled down it froze the valve? What would that entail is wrong with the head? Oh, so many questions. 

 

 

 

[Zed Head]

Turn the engine over by hand and see what the valves do.  If a valve stays down you should be able to pop the rocker arm off.  Might be able to grab the stem and spring retainer and pull it up.  Maybe it's something simple like a broken or weak valve spring.  Or a rolled over valve seal.  It's odd that you would have oil in the cylinder after sitting overnight unless a valve seal wasn't doing its job.  Get #1 up on the compression stroke and that should give the most space between cam lobe and rocker arm.

Hoping for damaged valve seal and no bent valve...

[ckurtz2]

Will check it out later today. Darnit just retorqued the valve cover. Here is a photo of the plug. Really at this point head has to come off anyways:/ 

 

 

[SteveJ]

I found this on someone's car about a year ago. Cylinder 2 was reporting zero compression. The valve was mis-adjusted and the lash pad eventually turned on its side.

 

[ckurtz2]

Mystery solved @Zed Head @SteveJ thank you both. You got it, Steve. Pulled the valve cover again and took a look. The rocker had jumped off the lash pad to rest on top of it. Not sure how I missed this obvious problem.  Anyways, popped it back in place, cleaned the spark plugs, and it now runs like a top. It is cammed and I checked the valve lash and it actually looked perfect for the stage one Comp Cam. Not sure how it popped off then? Must have happened on start-up yesterday, somehow. 

Any ideas on why this may occur? I will see if that ticking noise is gone for good once it's cold. I haven't heard it since during this process. It is possible that maybe the rocker had been working its way off for a while, and reached its final straw. Still no conclusions for the oil on plug. Could be that the rocker wash pushing valve at an angle. No smoke out tailpipe again.

People be wary of going straight to EFI for diagnosis, the loss of compression was throwing off the vacuum and AFM vane enough to cause woes in that system.

 

 

Edited October 30, 2023 by ckurtz2

[SteveJ]

I was just following @Zed Head's reasoning to a possible logical conclusion.

[Zed Head]

Thanks SteveJ, but my thought almost had a head pulled.  Keeping things simple until they need to get complex is the way to go.  Well done.

I'd guess that at 300 miles the valve guides are as tight as they'll ever be.    Sticking valves seems to be a not uncommon L series engine problem.  Might happen again.