Shaky Idle Issue/Low MPG

The Nissan 300ZX (Z32) general community discussion forum
tig40
Posts: 3
Joined: Fri Jul 01, 2022 2:46 pm
Car: 1992 Nissan 240SX S13

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Yo so recently purchased my 92 300zx Na and its got a pretty shaky idle. Feels like the whole car is shaking, not just vibrating. Idles at around 700-800 rpm, adjusted IACV to raise it a bit as it was pretty low before. No CEL, Changed spark plugs, cleaned MAF and replaced air filters. I also am averaging about 150 miles on a full tank which I feel like is pretty low. When I turn on the A/C the idle rises to about 1000-1100 and the shakiness goes away. When I rev the rpms to anywhere above idle it also goes away. I tried to clean and ohm my fuel injectors but for some reason my readings are all over the place and can't get a consistent value. Cars got 89k original miles and I just want to see if there are any other solutions before I commit to changing the injectors.


Daspyda
Posts: 78
Joined: Sun Feb 25, 2018 6:20 pm

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It's not the miles, it's the age and that ethanol is going through the injectors.

I just changed mine out due to similar symptoms, but when I drove a little while, turned off the car for a couple of minutes, it purred like a kitten. It's now as a new car.

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NolimitZ32
Posts: 7112
Joined: Fri Jun 27, 2008 9:07 am
Car: 91 AG2 2+0 TTMT swap/E39 BMW 540i6/E53 4.6is Dinan S3
Location: Houston, TX

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Its more age than anything. The ethanol is a much bigger problem for carbureted engines than it is for any fuel injected engines. The age is what causes the hoses and lines to break down and leave crap floating around in the fuel system.

OP, before doing anything to the injectors check your compression and perform a leak-down test. Much cheaper than servicing or swapping injectors. You can loan the gauges and manifold sets from Autozone. If your injectors ohm out and are operational it may be cheaper to have them cleaned rather than replacing.

Also does your CEL work? does it come on when you switch the key to ACC? Check the codes at the ECU to be sure.

Daspyda
Posts: 78
Joined: Sun Feb 25, 2018 6:20 pm

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Don't you have remove the plenum to clean them?

Might as well replace them while it's off?

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NolimitZ32
Posts: 7112
Joined: Fri Jun 27, 2008 9:07 am
Car: 91 AG2 2+0 TTMT swap/E39 BMW 540i6/E53 4.6is Dinan S3
Location: Houston, TX

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That all depends on the available funds the OP has to play with. Plenum removal isn't necessary for an injector swap, one can use the Dremel method. But to do it right its best to remove the plenum, replace the upper plenum gasket (cheap) and perform the coolant bypass. Servicing/cleaning injectors costs about $20 each. Buying new Phase 1 injectors (if you can even find them) costs $135 each. The Phase 2 kit w/adapters is $700. If the original injectors are within spec there is no NEED to change them. If funds allow then yes its best to replace the originals with phase 2 + adapter. But the difference between $120 and $700 or $800 is a sizeable one for many people, especially if you're hunting down an issue that may end up requiring a more sizeable investment to cure.

OP I missed the part about your readings being all over the place. Are you OHMing the injectors with the engine off at the contact or through the back of the plug while running? I know it seems pretty obvious, just want to make sure. How have you attempted to clean the contacts? If you're using the correct procedure to OHM them then corrosion would definitely play a role in odd readings and in rough running. If your idle evens out at slightly higher RPM like you mention then the culprit can be any of the following being at the tolerance threshold: Mechanical Compression, Electrical Limit MAF, Electrical Limit Injectors (failing coils), Mechanical Limit Injectors (dirty), Incorrect timing (CAS angle). Troubleshoot the things that can be tested first then start swapping parts otherwise the cost can easily get away from you.

The main point to all of this is to diagnose properly and not jump to conclusions even though a good portion of the time the conclusion may be right.


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