Tachometer relationship with TPS

[Zed Head]

Hey FW, that's a good idea but I think that the AAR is on the fuel pump power circuit. So if he takes the wire off of the starter solenoid and holds the key to Start, he should hear the fuel pump run and the AAR should get power. It does take a little while to close. But that could definitely be part of the high idle problem.

[FastWoman]

^^^ better idea

[johnny haywire]

Thanks guys, and duly noted. As soon as I can get someone to the garage with me, I'm going to test the AAR and the cold start valve. To test the valve, I'm just going to pull it and place it in a container and see if fuel is still coming out after the car starts (with someone else starting the car). Is this what you guys do?

One other odd thing I've noticed about this Z is that the throttle doesn't settle very quickly after a rev, especially when it is in its high-idle mode. Not sure if that's a clue to an issue with either of these devices.

Thanks again for all your input, it is very appreciated! I'll be sure to update the post once I've performed some tests.

[Wade Nelson]

All you ever wanted to know about the AAR valve.

http://www.classiczcars.com/forums/thread46090.html

Fast is correct, your 280 doesn't have an 02 sensor, my 280zx does. I had assumed ALL FI cars had an 02 sensor, it's a far better way to reduce emissions AND get optimum mileage out of a FI car.

Again, what the TPS does is tell the ECM "Hey, I'm at idle, I'll let YOU try and control the idle speed." If it's too far out of bounds there are mechanical adjustments on most. (A screw controlling an air bleed past the throttle butterfly)

Have you got a copy of the Datsun FI "bible?" It's available online, e.g.

http://www.xenons130.com/reference.html

It's a must-read.

[johnny haywire]

Many thanks for pointing me in that direction, Wade! I'm going to pull the bugger off tonight and give it the same run-through you did. I suspect mine is jammed wide open.

I've been reading the FI bible (the window has been permanently open on my browser for about two weeks), but didn't have a .pdf version, so this really helps (I bring my laptop to the garage while I wrench on the Z, but no wifi there). Looks like I owe you two beers, sir!

I'll keep you guys posted on what I find.

[johnny haywire]

Hi Wade,

I pulled the AAR last night and it was basically the same situation as with yours (thanks so much for the link to your post on the AAR, it really helped me understand how it works). So, my valve was stuck shut, I was able to pop it open with the pick, cleaned it out, did the freezer test, and it appears to work. A bit sticky still, though, so I'm going to clean it out some more before installing it. Also need new hoses, the split as I was removing them.

Next up...the cold start valve, which I think is my culprit. Looking at temps up to 104 today in Chicago, though, so I think I'll hold off on that for a few days...

Thanks again!

[johnny haywire]

Just a quick update - I bought some new hose, gave the AAR a better cleaning with some carb cleaner (opens and shuts freely now), cleaned the contacts and popped it back in. The car fired up fine, but with no real difference since it was around 100 degrees yesterday (and it was stuck closed when I pulled it, so I didn't expect much to change after seeing that). I didn't have time to drive it to see if the high idle issue was still there (it seems to like to hover around 1,400 RPM and occasionally drop to about 900).

I hope to test the cold start valve tomorrow and will keep you guys posted.

[johnny haywire]

And another update...tested the cold start valve today as per the FSM for 1978, checked out fine. Did a jump off the battery and she pumped some juice, appears to not be leaking (pulled it and ran the car with the CSV in a jar, and nada). I was able to take her for a ride afterward, and the idle seemed to settle a bit better (hovered more near 1,000 to 1,100 RPM after tweaking the air screw on the AFM and the idle screw), but still runs really rich and the throttle doesn't settle quickly, drops fine from high RPMs, but hovers around 1,200 - 1,400 and takes maybe 5 seconds to settle to normal idle settings. It was in the low 90s today, so I don't think any of the cold start components came into play. I'm thinking my AFM is maybe the culprit? Going to start testing that next...any tips are welcome. I don't want to start cheating the AFM to dodge a bad sensor, etc.

I also replaced some damaged vacuum hoses today. I did my best, but there has been a butcher in there (some hoses were not connected on the battery side) and I'm not sure everything is hooked up correctly, still researching the FSM with regard to the routing and vacuum readings. The "ET" section seems to have the most information, but not much. This is some uncharted territory for me. My heater does not work presently, either, thinking all those vacuum lines need replacing, and a possible flush of the heater core.

Anyway, we're getting there...

[Zed Head]

Engine RPM is mainly controlled by air supply. Which is mainly controlled by the throttle body. The AAR bypasses the throttle body when it's open, that's why it was a good place to look. Leaking hoses can let air in that will increase RPM, as long as there is fuel available, so any hose attached to the intake system should be checked. The PCV system can let air in from the crankcase, if there is air leaking in to the crankcase. For example, through the dipstick tube, valve cover gasket or oil fill hole.

Can you get the idle to hang high while watching the engine with the hood open? A weak throttle return spring or sticky throttle blade can hold the idle up. That sounds more like your problem since the idle eventually drops down. Watch the throttle blade lever and see if it's dropping back to its stop quickly. The AFM just responds to the air flowing through the throttle body, so it's unlikely that it would cause a high idle.

[FastWoman]

Your heating and air conditioning controls are all vacuum driven. There's a little vacuum hose that runs from that pair of magnet valves on the passenger wheel well, through the firewall, and to the mode selector switch (e.g. off, vent, a/c, heat, etc.) in the dashboard. If that leaks or doesn't have vacuum, the vacuum valve on your heater core will never open. That's probably why you don't have heat. Fortunately you don't have to worry about that for a while! If I'm not mistaken (and I might be), vacuum isn't needed to run the A/C. You may as well cap off that little vacuum fitting right beside the larger power booster vac hose near the backside of the intake manifold, until you get your engine running right. Then you can deal with the heating system later.

[johnny haywire]

Thanks again for the great responses, guys! This thread has been a MAJOR source of help for me.

The idle used to hang high whether I was in the car or not, but it didn't really do it yesterday in the garage, maybe due to the heat. Maybe due to all the sensor/component connector clean-up I've been doing? I'll try again this afternoon and have a look at the return spring and the throttle blade and let you know what I see. I should probably open that up and clean it out a bit. I did pull the AFM at one point last autumn and clean it out with AFM cleaner, but there wasn't much effect, if any.

I have replaced all the vacuum hoses on the driver's side of the engine as well as the PCV, with the AAR hoses being the most recent and the last of them, I think. (All others were done right when I bought the car before it made the trip from LA to Chicago last year). And you are right, I should leave the heater vacuum issue for the future (good call!) and get the engine running right first. So many little things to get caught up on as I go along.

One thing I noticed while doing the CSV test yesterday was that my outer boot to the air filter was in good shape, but the one between the AFM and the throttle body seemed to have cracks around the camping points. This could be a source of extra air sneaking in, right? I'll pull it and inspect. It's probably original and getting pretty hard. The outer boot toward the air filter seemed supple, though.

Thanks again for the pointers! I'll be sure to update once I get over to the garage.

[johnny haywire]

Okay, another update, had a chance to pull the AFM and clean it out. The "cracks" on my boot were actually the part numbers, and both boots are good, but...someone has been into the AFM. Damn!

Clean as a whistle inside, flap moves freely but slams shut, so, the "return to zero" shock absorber spring is not set correctly. To make matters worse, the edges of the black cover is in pretty rough shape and has been siliconed on pretty tightly. I've read through the Atlantic Z pages on the AFM adjustment and calibration and that pretty much looks like what's in store for me.

I've decided to leave it alone for today and try to carefully remove the black cover tomorrow. I can see little parts on the edges are missing as it is, so going to try and remove it as gently as possible.

I wonder if MSA would still accept this one as a core, even though it's been messed with and the cover is damaged?

The numbers on the black cover are:

Part No.: A31-601-000

Lot No.: 75236

L28

Curious if it's even original for my '78...