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I'm trying to diagnose a few things on my 75 280z 2+2, and I'm wondering if they might be related. Any help would be appreciated. Also, does anyone know why the section of my camshaft above cylinder #2 is red? Video.

 

In summary, I can't seem to get power out of my engine. It used to run "great", but now I can barely get the car over 30mph.

 

  • I ran a compression test, fearing a blown head gasket, but though the readings aren't great, I would have expected to get over 30mph with them (1: 170, 2: 156, 3: 167, 4: 169, 5: 160, 6: 165). That #2 cylinder seemed especially low, so I ran it with a tsp of oil in the cylinder, and no change - still 156.
  • My vacuum is measuring low at about 13 in of Hg. I replaced all the old vacuum tubing, but it's still running low, even with most subsystems' vacuum lines blocked off.
  • Fuel is spraying well out of all injectors.
  • Fuel pressure is at 38 psi with the fuel pump on, but engine off, then drops down to ~30 psi when idling.
  • Using a timing light, I adjusted the distributor til the engine was at exactly 10 degrees by the timing mark.
  • I lashed the valves back in the day at cold, but I definitely hear a clicking from the engine that I don't love. Based on that and everything above, I imagined that I must have just screwed up the lash, so I opened the valve cover and checked, but no, the lash was exactly where I left it.

 

So I don't know. Any pointers, ideas, or other debug steps would be appreciated. And yeah, if anyone knows why that section is red on the camshaft, I'd love to know! Thanks everyone.

 

Valves in action

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#2 is low but it's within spec range, 10%.  The engine should run fine just with maybe a lumpy idle due to the imbalance.  You should definitely be able to go faster than 30 mph.  The red on the camshaft is just factory paint, it has no meaning.  Your rocker arm play is normal, the lobe was on the base circle (off the lobe), with no pressure on it.  The others probably all do the same.

 

Is it a manual or an automatic?  When did you last give it a tuneup?  Do you drive it often?  Is it possible that there is water in the fuel tank, from sitting long periods between driving?

 

Doesn't seem like a major engine problem.  Probably some minor electronic or fuel problem.

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I agree with NewZed.  Did you verify that 0 degrees on the crank damper/pulley is, in fact at TDC on cylinder 1 compression stroke? The elastomer pulleys on these old engines tend to crack with age and spin or move on the crankshaft. If this has happened, your timing could be by a lot.

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Thanks for the suggestions!

 

@NewZed, automatic, 4 speed. The water idea is very interesting. I was driving it daily at the beginning of this year, but I stopped doing that around February (working from home). It's also right around then that it stopped delivering power... The rubber on this car is real bad, and it wouldn't surprise me at all if the gas tank was leaking.

 What would be included in a "tuneup"? Within the past 6 months I've done everything mentioned in the original post, plus replaced engine+trans oil, oil filter, fuel filter, thermostat, flushed engine coolant + peak cleaner  + new coolant, new trans auxiliary cylinder, new trans fluid, new brake fluid, bleed all those lines. I've kinda been just fixing and replacing stuff as I'm able in a attempt to get it running close to "smooth".

 

@rossman, I was curious about this as well. What's the best way to check? At 0 on the timing mark, the camshaft lobes in cylinder 1 are in "rabbit" ears configuration, both pointing up, symmetrically. But I guess that would mean that just the valves are in sync, not necessarily the pistons?

Edited by daughter.and.datsun
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@NewZed  Is there any good way to check if water is in the tank? My sender float is likely bad, so I was going to drop the tank sometime soon to investigate that. Any notoriously bad gaskets or anything I should look out for?

 

Again, thanks to you both. This is an interesting line of investigation!

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I don't see spark plugs, distributor cap, rotor.  Usually the first tune-up parts people think of.  

 

Here are some things to check that might offer a clue.  Is the transmission shifting through the gears as you accelerate to 30 mph?  What is the tachometer needle doing?  Is it behaving normally, following engine RPM, or does it jump around?  Do you think that the engine is running well, besides the lack of power?  In other words, does it sound smooth or does it sound like it is missing or stumbling as you try to accelerate?

 

Give more details on what, exactly, the engine itself is doing as you try to get to 30 mph and what happens when it stops accelerating as the speedo needle stops climbing.  What is the transmission doing?  Do you think it's functioning properly?

 

Engine noise and tachometer needle are good indicators of engine performance.  Exhaust smell is also a clue.  There are two common problems with the old cars, the ignition module and the ECU.  The ignition module will typically start shooting too many sparks when it goes bad and the tachometer needle will start acting weird. and the engine will run poorly.  The ECU's tend to run very rich as they start to go bad, causing a very gassy smelling exhaust, then they die completely.

 

To rossman's point, if your timing is retarded the engine will be way down on power.  But if it started running poorly before you adjusted timing you'd have to wonder how the timing got retarded, unless something broke in the distributor.

 

One way to check if your damper has slipped is to match the notch and groove on the timing chain sprocket and see if you're close to zero on the timing mark.

 

There's a bunch that ran through my mind...

 

If the engine is running well, it might just be your transmission.  Try using the shift lever to stay in the lower gears and see if things are different.  Separate engine issues from transmission issues.

Edited by NewZed
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Could also be a partially plugged fuel filter.  Even though you changed it, there could be a bunch of junk in the tank that is slowly getting sucked thru.  If this is what's happening then it would get plugged up faster with a low fuel level.  Drain the tank to find out for sure, this would also tell you if it's full of water.

 

As NewZed mentioned, a few more details on exactly what is happening may help us hone in on the problem.  For example, is the engine still revving to 6000 rpm but has no power, or is it sluggish and won't rev?  There are many things could be causing your problem but until you start doing some diagnostics, you're not going to figure it out.  

 

Edited by rossman
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@NewZed Ah! Forgot to mention new plugs (which are all receiving spark) and a cleaned and inspected/measured distributor cap.

 

The engine seems to be functioning well. I'll post a video next time I give it a go. It seems to move through gears smoothly (I last took it up to about 3rd trying to get above 30mph on a flat road), and the tach moves smoothly through RPM, but it's just as if the output power curve has been adjusted down to near nothing. Things sound smoothish too (aside from a singular rev-matching clicking sound coming from around the engine). 

 

All good suggestions, I'll try them out.

Re: transmission, I used to be able to get up to above 30mph in first gear, but now I can rev up pretty high keeping it in second and only make it to 20mph. I have to take it to third to gut up that 30mph mark. I'm ready for the races, for sure 😊 

Re: timing, I suspect that the owner before me swapped in a whitehead performance sprocket gear or something similar, as well as the chain, because I can't find any bright link on the chain. So I don't know what's going on there, but I also haven't touched it between the working time and the not working time. This is something I've wondered about - if my timing light measures 10degrees, does that mean that whatever's going on with the dampener or chain tension, it's firing at 10degrees? Or does it simply mean that spark is firing when the mark is at 10degrees, but who knows what's actually happening with the pistons?

 

No need to respond to any of that - I truly appreciate the pointers you all have given so far! Thanks!

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Check the simple stuff like the air intake.  Make sure the throttle blade is opening completely.  Check the intake system for mouse nests, they work fast.  Make sure the filter is not clogged.  Take the black cover off of the AFM and make sure the counterweight/vane is moving freely as you rev the engine.  The AFM vanes have been known to bind. 

 

The only thing stopping an in-tune engine from revving and producing more power is air flow.  Your symptom sounds kind of like an air flow problem.

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