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Stumbles at 1800-2200 RPM

Old 06-23-2014, 10:22 AM
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Stumbles at 1800-2200 RPM

I have a 1976 280z, it runs and drives well, except it has this annoying problem where it runs very rough between 1800 and 2200 RPM. Above and below this range its fine...

Recent maintenance includes cleaning all the fuel injector contacts, doing the full suite of tests on the EFI ECU (from the other EFI book that is not the FSM, but refers to it). Cleaned electrical contacts on the air flow meter... misc other stuff.

Any ideas? My thought was that the AFM might have a sticky spot, but when I had it out to clean up the contacts it moved quite easily.
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Old 06-23-2014, 11:03 AM
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throttle position sensor/ switch

It could be your TPS, it has 2 switches, one that monitors Idle, the other Full Open, you just might need to adjust it so it is seeing the throttle idle switch open when you hit the pedal. The manual (fsm) will walk you through it.

I found these, looked pretty helpful

TPS Throttle Position Sensor Adjustment (Pre)Guide -'76 280Z - 6 Cylinder Z Forums - HybridZ

TPS Adjustment
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Old 06-23-2014, 12:38 PM
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Originally Posted by PredatorZ View Post
It could be your TPS, it has 2 switches, one that monitors Idle, the other Full Open, you just might need to adjust it so it is seeing the throttle idle switch open when you hit the pedal. The manual (fsm) will walk you through it.

I found these, looked pretty helpful

TPS Throttle Position Sensor Adjustment (Pre)Guide -'76 280Z - 6 Cylinder Z Forums - HybridZ

TPS Adjustment
k... thanks for the reply... I didn't have a buddy helping me so I could not observe what the TPS is doing and correlate that to the tach. I saw that it moved properly and the continuity checked out at the ECU. I will clean it up some more and see if I can set it... maybe it's not coming off the idle side quick enough and leaving it a bit rich.

I have a few more hoses to replace as well, so a vacuum leak might be another variable in the equation.
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Old 06-24-2014, 11:19 AM
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So good call... I showed my 9 year old daughter how to read the tach and keep the throttle at 1400... Meanwhile I took a look at the sensor and it was still pegged on the idle side. I had her keep giving it gas and it didnt come off the idle side until around 2800..

Will get it adjusted tonight and see if that fixes the issue.

The daughter and I also replaced one of the two vacuum hoses that were looking worse for the wear.

Will report back here...

PS: "Hey, if Mom asks if you were in the Datsun pushing on the gas pedal while it was running, say yes, because Dad asked me to. If she doesn't ask, then I'm not sure we need to tell her..."

Plan to turn the TPS in its bracket until it comes off the idle side at 1400... anyone know what RPM it should hit the WOT side?

Last edited by Pez; 06-24-2014 at 11:23 AM.
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Old 06-25-2014, 09:42 AM
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So we got the TPS adjusted for the idle side, and it still stumbles in about the same spot. I will go through some more of the vacuum lines and see if I can find anything else.
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Old 06-26-2014, 01:35 PM
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It might be that your vacuum advance isn't working. Very common in an old dizzy. Have your daughter rev it up with the vacuum hose to the dizzy off and feel if there is any suction. If there is then pull the cap and make sure you have movement in the breaker plate. If you do then the vacuum pod off the dizzy. Have your daughter repeat the revs and see if the pod retracts the rod.
Make you feel great when she tells mom "I was helping Daddy"
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Old 06-27-2014, 07:48 AM
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Originally Posted by theramz View Post
It might be that your vacuum advance isn't working. Very common in an old dizzy. Have your daughter rev it up with the vacuum hose to the dizzy off and feel if there is any suction. If there is then pull the cap and make sure you have movement in the breaker plate. If you do then the vacuum pod off the dizzy. Have your daughter repeat the revs and see if the pod retracts the rod.
Make you feel great when she tells mom "I was helping Daddy"
Sweet, I just learned a great deal. Will check it out this afternoon.
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Old 06-27-2014, 08:48 AM
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hm... how easily should the rod move? there's suction at the line, and the rod for the vacuum pod moves, but it moves with a lot of difficulty.

I guess any vacuum leak in the system would cause the vaccum advance to not function, as it would not be able to build negative pressure...

Edit: ok, the rod moves maybe 1-2 mm max when the car is at it's 'stumble'

Last edited by Pez; 06-27-2014 at 09:00 AM.
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Old 06-27-2014, 09:09 AM
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If you can

If you can clean and lube the advance mechanism, that might fix it, been a long time since I had a car with a mechanical advance on the dizzy. You might consider upgrading to an s130 type distributor , not sure that would cure your problem, but it is a more reliable design. Make sense with everything else you have done, good call by TheRamZ, I would have not thought of that.
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Old 06-27-2014, 11:34 AM
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Originally Posted by PredatorZ View Post
If you can clean and lube the advance mechanism, that might fix it, been a long time since I had a car with a mechanical advance on the dizzy. You might consider upgrading to an s130 type distributor , not sure that would cure your problem, but it is a more reliable design. Make sense with everything else you have done, good call by TheRamZ, I would have not thought of that.
I know it's not working... and the vacuum advance it pretty hard to find.. no luck so far.. I think the 77 will work for my 1976 California, but not sure.

May end up getting a new dizzy for the vac advance alone.
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Old 06-27-2014, 12:39 PM
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should fit

Just looking at the 77 to 79 FSM's, the 79 details out about an electronic module in the distributor, that was the 1st I found, but might have missed it. Nismo or Fricfrac would know for sure, hopefully one of them will read this. Looks like the mechanical advance was on all of them, just a better pickup on the later models. Looked like greasing the parts on the advance might just fix them, if you have 2 to work with, you probably have what you need to fix it.
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Old 06-27-2014, 12:51 PM
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manual or automatic ?

Found this , not sure if it applies to your car, good info if it's true.

"Something else to watch for is that you've got an automatic... That means that you've got the dual pickup distributor and it's accompanying two input switchable curve ignition module. The system switches timing curves depending on engine temperature. Cold engine = advanced, warmed up = retarded. Controlled by the temperature switch located in the thermostat housing (the two wire device next to the single wire device which drives the temp gauge on the dash)."

from - 260Z to 280ZX distributor
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Old 06-28-2014, 11:20 PM
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Originally Posted by PredatorZ View Post
Found this , not sure if it applies to your car, good info if it's true.

"Something else to watch for is that you've got an automatic... That means that you've got the dual pickup distributor and it's accompanying two input switchable curve ignition module. The system switches timing curves depending on engine temperature. Cold engine = advanced, warmed up = retarded. Controlled by the temperature switch located in the thermostat housing (the two wire device next to the single wire device which drives the temp gauge on the dash)."

from - 260Z to 280ZX distributor

That is correct. Some have that built in dual ignition control.
On this type, sometimes one of the curve circuits goes bad. The 76
may indeed have that set up. It will run bad cold and run good warm,
Or it will run good cold and bad warm. On passenger kick panel I believe.
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Old 06-29-2014, 05:09 PM
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I don't think 1-2mm is enough. You should be able to move the breaker plate by hand. This is what moves the timing advance until enough rpms can advance it. Some of your vacuum could be getting robbed from a leak somewhere.
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Old 06-30-2014, 06:51 AM
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If your advance is sticking, more than likely that is the issue. Did you pull the plugs and see what they look like?? Do you have a timing light?

http://www.rockauto.com/catalog/rafr...&parttype=7108

Last edited by Skully; 06-30-2014 at 06:56 AM.
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Old 07-01-2014, 12:42 PM
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Originally Posted by Skully View Post
If your advance is sticking, more than likely that is the issue. Did you pull the plugs and see what they look like?? Do you have a timing light?

RockAuto Parts Catalog
I don't yet have a timing light or the skills to use one... I should have the advance before the holiday.
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Old 07-01-2014, 01:18 PM
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Originally Posted by Pez View Post
I don't yet have a timing light or the skills to use one... I should have the advance before the holiday.
Would recommend a cheap timing light even. Skills..you don't need any to use it. Clean the timing marks on the engine so they are visible, I usualy paint mine and then sand down the metal, so the numbers stay painted. Find the timing notch on the pulley, put a little dab of paint on it too, white is good. As far as the light goes, hook it up to the battery, clip the clamp onto spark #1, it will have an arrow that points TO SPARKPLUG, so orientation matters. point the light to the timing plate and it will flash every time crank pulley hits the timing corresponding on the #1 timing cycle. Its super easy, HIGHLY recommended!
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Old 07-02-2014, 05:04 AM
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Originally Posted by Skully View Post
Would recommend a cheap timing light even. Skills..you don't need any to use it. Clean the timing marks on the engine so they are visible, I usualy paint mine and then sand down the metal, so the numbers stay painted. Find the timing notch on the pulley, put a little dab of paint on it too, white is good. As far as the light goes, hook it up to the battery, clip the clamp onto spark #1, it will have an arrow that points TO SPARKPLUG, so orientation matters. point the light to the timing plate and it will flash every time crank pulley hits the timing corresponding on the #1 timing cycle. Its super easy, HIGHLY recommended!
Dig it... will grab one after the holiday.
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Old 07-07-2014, 09:40 AM
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Originally Posted by Skully View Post
Would recommend a cheap timing light even. Skills..you don't need any to use it. Clean the timing marks on the engine so they are visible, I usualy paint mine and then sand down the metal, so the numbers stay painted. Find the timing notch on the pulley, put a little dab of paint on it too, white is good. As far as the light goes, hook it up to the battery, clip the clamp onto spark #1, it will have an arrow that points TO SPARKPLUG, so orientation matters. point the light to the timing plate and it will flash every time crank pulley hits the timing corresponding on the #1 timing cycle. Its super easy, HIGHLY recommended!
Indeed it was super easy. I found that it was nearly off the scale in the 20-30 range. I adjusted the idle to 800 rpm with a warm engine and then set the timing to 7 degrees.

I put in a remanufactured dizzy that came with a new vacuum advance. I could not find the actual vacuum advance by itself, so I got the entire unit.

Over the holiday I drove it to the lake place, parts in hand. I replaced the vac advance by itself and it didnt correct the stumble. Then I replaced the entire dizzy. I set the timing again at 7 degrees.

It's still stumbling between 1800 and 2000.

Today I had the day off so I monkeyed with it some more, I found the #5 and #6 injector connections were reversed, but still didn't solve the issue.

I have all the parts in hard for a full injector rebuild with the blocks, plates, screws injectors and connectors. I'm heading out of town for a few days, so that will need to wait a week or so.

Any other suggestions along the way would be appreciated. When I get back from vaca I want to look into getting some gear for a vacuum pressure check as there are still some old hoses and connectors.

Thanks for the help so far.
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Old 07-07-2014, 09:58 AM
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common pulse

These cars have a common pulse, so all the injectors actually fire at the same time, so you really can't get them mixed up. You might check if you are getting one mis firing, you could just have a bad connection, those old efi connectors get very brittle. Fricfrac (Kurk) has a kit to replace the efi stuff, along with a new maf connector and tps too. I did a little writeup in my 81zx thread, ill link it, was well worth it, mine were in really bad shape, the new ones are all cool new tech.
https://www.zdriver.com/forums/280zx...y-39110/page2/
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Old 07-07-2014, 10:19 AM
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Get one of these. http://www.ebay.com/itm/Brake-Fluid-...r-Kit-Adapters
You can test your injectors individually by hooking up a 9 volt battery to one at a time. No clicky no worky.
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Old 07-07-2014, 10:23 AM
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Originally Posted by PredatorZ View Post
These cars have a common pulse, so all the injectors actually fire at the same time, so you really can't get them mixed up. You might check if you are getting one mis firing, you could just have a bad connection, those old efi connectors get very brittle. Fricfrac (Kurk) has a kit to replace the efi stuff, along with a new maf connector and tps too. I did a little writeup in my 81zx thread, ill link it, was well worth it, mine were in really bad shape, the new ones are all cool new tech.
https://www.zdriver.com/forums/280zx...y-39110/page2/
Yea, I had read that as well. I can hear all of them click with the ear/screwdriver test. I've cleaned all the electrical connectors, using a file and spray contact cleaner on the male side, and just contact cleaner on the female side. They are reasonable inexpensive so I ordered them along with the injectors.

Side note that seems related, does anyone know a good source/substitution for the AFM boots? MSA wants $90 for a set of them, plus they have been providing crappy service lately (IMHO).Seems like I can get 3" silicone hose at auto zone for a few bucks.

Motorsport! Boot, AFM to Throttle Body, 75-4/77 280Z - The Z Store! Nissan-Datsun 240Z-260Z-280Z-280ZX-300ZX(Z31/Z32)-350Z-370Z Parts
Motorsport! Boot, Air Cleaner to AFM, 75-78 280Z - The Z Store! Nissan-Datsun 240Z-260Z-280Z-280ZX-300ZX(Z31/Z32)-350Z-370Z Parts
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Old 07-07-2014, 01:18 PM
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Pez I got a silicone 45ish degree boot off ebay when I was getting intercooler plumbing, not going to lie I don't know the size it was but it fits great.

Another thing you can try if all else fails. Moving the board in the AFM to make new wiper arm contacts on the resistor strip. I had a stumble as well, and I did this (amongst other things tho) but it might be worth a try.
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Old 07-15-2014, 07:10 PM
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Thanks Skully, can you elaborate on what you mean? You mean moving the board up or down so the wiper strikes a new path across it? or do you mean turning the unit so it manually makes it run rich / lean?

I will re-inspect the AFM board / wiper to see if something is obviously worn.

In other news, I finished my injector rebuild today and was quite pleased. There are things running very well in this car now, but things not yet exactly right. I think I'm still chasing a vacuum leak. I looked into getting a vac tester today and it was $55 USD / day to rent. I didn't want to expand my scope on the injector rebuild so I declined getting it.

One question I have related to timing. I set the timing to the FSM recommended 7 degrees, but there seems to be a consensus on the internets that as a car ages, the timing should be advanced a bit further, some even modifying the mount on the bottom of the dizzy to advance beyond the scale. What is a general guideline for a 1976 with 133k miles? 15, 20?

My thought is to adjust by fives until it sounds better and then fine tune by single degrees... then test drive...
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Old 07-17-2014, 06:52 AM
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Originally Posted by Pez View Post
Thanks Skully, can you elaborate on what you mean? You mean moving the board up or down so the wiper strikes a new path across it?
this part is what I did. Just be careful to move the board straight and not move it side to side.
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