Hesitation at 3000 RPM/Oil Leak?

[KDMatt]

I've finally, sort of, put my finger on a peculiar acceleration quirk I've noticed with my Z. Last weekend I ran the engine in my 280z for the last time (presumably) until spring, and I decided to put her through her paces by slowly pressing in the accelerator further and further (why not?) ... What I noticed is that the engine seemed to "hang" at about 3000 rpm. I kept pushing in the accelerator slightly, it stayed at 3000 rpm for a bit, and then shot up to 4500 as though playing "catch up" with itself.

I don't believe this to be a tachometer issue since the car was stationary, and I'm pretty sure I would have noticed the engine speed climbing.

... On a side note, I've also noticed that if the car sits for more than a few days it'll burn off some oil when I run the engine: the other reason I was pushing the engine to high revs.

Any ideas? ...

[BuDavid]

the oil burning matter, does it burn oil when you start the engine for few moments then it stops when it gets warm? if so it might be due to the valve's seals, the pass oil when the engine is cold and stop when they get warm due to expansion..

u might notice smoke from oil burning also when decellaration from high RPM also in worse cases.. small process fix it if so..

sorry but I have no idea about the hesitation..

[KDMatt]

the oil burning matter, does it burn oil when you start the engine for few moments then it stops when it gets warm? if so it might be due to the valve's seals, the pass oil when the engine is cold and stop when they get warm due to expansion..

u might notice smoke from oil burning also when decellaration from high RPM also in worse cases.. small process fix it if so..

I burns oil when it's cold, particularly when I race it. The more I race the engine, the more oil it seems to burn off. When the engine is warm however, this is not a problem.

So, you think that could be related to bad valve seals eh? Is it going to be devastating to the car to let it sit, unused, for six months in cold minnesota winter?

sorry but I have no idea about the hesitation..

Don't sweat it.

[BuDavid]

I

So, you think that could be related to bad valve seals eh? Is it going to be devastating to the car to let it sit, unused, for six months in cold minnesota winter?

Don't sweat it.

u can leave it temporarly if the smoke accures when u decelarate coz by that time u'll have very bad seals..

ask more about it, maybe I'm wonr.. better to asked experienced mechanic.. good luck

[LanceM]

I'd vote for valve seals too, really not that hard to change with engine in the car, might make a good winter project since it is going to sit anyway.

[Ed]

If it's going to sit in your garage all winter I'd vote for a valve seal change. And about the hesitation. Were you just watching the tach and the needle jumped or did you listen to the engine?

[KDMatt]

I'd vote for valve seals too, really not that hard to change with engine in the car, might make a good winter project since it is going to sit anyway.

What's involved in replacing the valve seals? (I don't have an FSM just yet, it's on my to-buy list). Since I don't know all that much about engines just yet I'm just going to assume that I'd need to remove the cylinder head, correct?

Would it be wise, in the mean time to just put some gas in it and run the engine every now and then? ... Perhaps even adding some Oil "Stop-Leak" stuff?

If it's going to sit in your garage all winter I'd vote for a valve seal change.

Actually, it's sitting outside (covered at least), which is not to my advantage, I know, but I've really got little choice about it.

And about the hesitation. Were you just watching the tach and the needle jumped or did you listen to the engine?

Both.

I think my injectors might need to be replaced. If they were backed up with gunk it would make sense that the fuel flow would be restricted and that, all of a sudden, the increasing pressure of more fuel trying pass through as I opened the throttle more would cause a sudden jump in rpm, right?

You guys have been a tremendous help, and any more ideas you can put forth are always appreciate.

[KDMatt]

Hey again folks, I've done some research since last post, and I'm beginning to map out a winter/spring break project for my car now. (A friend of a friend is a big Nissan enthusiast and is going to graduate soon from... UTI is it? So I may be able to get professional grade help with this one.)

Here are the things I'm going to replace in the head:

Seals (of course)

Valve Seats (in favor of the premo/hardened ones)

Valve Guides (again, in favor of the premo ones)

Springs (... they're supposed to be replaced every 25k miles or so, right?)

... and perhaps even the valves themselves?

I remember reading something about seats and valves having to "match" each other or something to that effect? Well, anyway, from where I'm sitting at this particular moment in time, I figure that if I'm going to replace the seats, it's probably also wise to replace the valves themselves.

I might also, possibly, maybe... look into a new headgasket... since... well... that's another one of those since-I'm-in-here-this-far projects I was warned about from you folks after I bought the car.

I have some questions about head removal though... since I can understand that it would be very, very easy to f--- up the cam/valve timing... Is it possible to return the engine to the TDC position with it still in the car, attached the drivetrain, etc. or is it more realistic to just be very careful in removing the head? (i.e. taking careful note of the position the cam is in and just making sure that when the head is bolted back on that the cam matches my notes/pictures)

Any more input from you guys is, of course, appreciated.

EDIT: I think I might have spoken too soon... Replacing the seats would be a very involved project... one perhaps best left to a machine shop at a later date... so I think I'll nix that from my to-do list for this project.

[LanceM]

OK,

Long list

If you pull the head you WILL have to replace the head gasket,they are not (typically, some high performance ones are but not a stock datsun) reuseable . Since you are driving a 280 I would think but I may be wrong but you probably already have hardened valve seats. Valves and valve seats are ground to match so no need replacing valves unless they have already been ground past spec or are worn past spec. If you have the head off valve guide replacement is a good thing. If the springs are originial or don't meet spec replace them, in a normally driven car valve springs last forever, never heard of the 25K thing...

When removing the head the best way is to crank the engine to TDC before doing anything else. Be sure to block the cam chain tensioner before removing the cam sproket other wise you will have to pull the front cover to fix it. Best money spent here would be for a FSM, factory service manual, often on ebay, it tells you all you need to know.

[KDMatt]

OK,

Long list

If you pull the head you WILL have to replace the head gasket,they are not (typically, some high performance ones are but not a stock datsun) reuseable . Since you are driving a 280 I would think but I may be wrong but you probably already have hardened valve seats. Valves and valve seats are ground to match so no need replacing valves unless they have already been ground past spec or are worn past spec. If you have the head off valve guide replacement is a good thing. If the springs are originial or don't meet spec replace them, in a normally driven car valve springs last forever, never heard of the 25K thing...

When removing the head the best way is to crank the engine to TDC before doing anything else. Be sure to block the cam chain tensioner before removing the cam sproket other wise you will have to pull the front cover to fix it. Best money spent here would be for a FSM, factory service manual, often on ebay, it tells you all you need to know.

Right, okay... I shouldn't lose sight of what I'm really striving for here... I'll cross the new valves and seats off of my list and, add, in their place, a new headgasket.

... and I was under the impression that the N42 was designed to take leaded as well as unleaded gas, and therefore had softer seats? ... N47 on was strictly unleaded from what I'm told.

Actually, I'm looking at my VB catalog... it looks like a headgasket set comes with valve seals... is that right or am I just hallucinating? :stupid:

[LanceM]

Unleaded heads had no seats, it wasn't until after the removeal of lead from gas that heads had hardened seats installed. It's been a long time ago but I seem to remember that by 73 a lot of American cars were unleaded only... When you get the head off it will be easy to see if it has hardened seats in them, there will be a metal ring that is the seat, it probably won't be perfectly flush with the chamber, no seat no ring.

Yes the VB catalog shows all of the gaskets/seals required for a head rebuild, including valve seals.

[DatsunZsRule]

Correct KDMatt, the 75-76 heads came with softer bronze valve seats for use with leaded gas. I wouldn't cross that off your "to do list" because it sure sounds to me that the Valve Seats are the cause of your problem. If it were a head gasket problem then she would be smoking like crazy all the time. even when warm. If your not comfortable doing this job and don't have the tools required to do it, then I guess the best thing for you would be to ask that "friend of a friend" and see if he would be willing to help you out.

As for the Valves and Valve Springs- I don't know who told you that they need to be changed every 25,000 miles but thats False. Like Lance said "in a normally driven car valve springs last forever" Just inspect there condition when it's taken apart.

Best of luck.