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Rough idle and stalling

Old 10-08-2015, 05:16 PM
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Rough idle and stalling

Hi, I've recently purchased an 85 NA and it likes to stall on startup and on stumbles when coming to a stop.

The previous owner installed a 3" cat back from MSA. I'm assuming the idle issues are due to lack of back pressure. I'm considering switching out to the 2.5" MSA with an extra resonator to take care of some of the raspiness higher in the power band. I figure with the extra resonator and smaller exhaust it should take care of it.

Do you think this would help with the idle issues, or do you think I should look elsewhere first?

Thanks
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Old 10-08-2015, 09:48 PM
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what on God's green earth makes you think higher backpressure is a good thing. the idle problem is probably that way over engineered idle crap hanging off the plenum on the driver side. I threw mine away as have many others. Have you done normal tune up things. fuel filter, new plugs, new wires, new dizzy cap. check timing. it isn't black magic you need some knowledge ability. 30 year old cars need help.

Get your battery load tested. Be sure you have clean, tight corrosion free terminals on both ends of your battery cables. Be sure the ground (negative) cable goes to a bolt into the frame before going to the starter. grounding through the starter is not a reliable connection. Z's don't like low voltage. causes the electronics to act funny. it is possible to have enough amps to crank but not enough voltage to run the electronics. If you have one size fit all cheapo clamp on terminals they are a problem waiting to happen. Usually on a rainy night around Oh Dark Thirty.

Clean the connectors for the maf or afm, ecu and tps. Deoxit by CAIG is probably the best connector cleaner on the market. spray with CorrosionX after cleaning and before putting together this will help prevent any further corrosion. DON'T USE DIELECTRIC GREASE.

Replace outer tie rod ends, and ball joints. replace the bushings with poly (don't forget to lube them as directed unless you like squeaky things). New shocks. Get new boots for power steering. all those things will make you think you are driving a different car. doing them piece meal is a waste you won't see a great improvement until you do the complete job. You need to get the car realigned after messing with the front. Have your tires in good shape and rebalance. Rear bushings nice too but more work and you won't notice as great an improvement. Rear shocks also because if the fronts are gone so are the rears. if car squats when you jump on the gas the rears are gone.
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Old 10-09-2015, 02:26 PM
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As for why I think higher back pressure may be a good idea is that lack of back pressure can causes issues with scavenging at low rpm. Also from what I've seen most people dont recomend over 2.5" exhaust for the NA. I'm assuming that this recommendation originated due to some kind of issue like this.

I suspect the cluster of over engineered idle crap has already been removed, as their is a wire harness not connected to anything on the driver's side of the plenum, and it looks relatively un-clustered. (As un-clustered as any car with vacuum lines can anyway).

I also suspect that it may be due to a vacuum leak, so I'll be checking for that this weekend.

Haven't pulled the plugs yet, but the cap and wires are new, so I expect the plugs to be fine, but will check. Will check the negative cable frame ground, and fuel filter. Battery is ~a year old, and looks to be in good shape, but I will load test it.

Will check and clean the maf, etc.

It has new tie rod ends, and poly everything. Has Bilsteins on it. Fronts are good, but rears need to be rebuilt. The ride is quite good. The previous owner took good care of it.
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Old 10-10-2015, 09:48 AM
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don't know where you picked up the word scavenging but it is normally used when speaking of two stroke cycle engines not 4 stroke cycle (your common spark ignition engine). bigger the pipe the better. the only reason people speak of 2.5 inch is that was the stock dimension. Vacuum leaks definitely the bane of a fuel injected engine. One test is to use an unlit propane torch (valve open) and wave it around where there might be a leak and see if the engine smooths out (ie getting more fuel). Also examine closely the intake air system. People have reported finding cracks in the rubber. air going in that is not metered by the MAF causes the computer to generate erroneous signals. Blow out the wells that the plugs sit in - they are great collectors of all the small screws that have been dropped over the years and don't play well inside the cylinder. Make sure the wires to the plugs are in the correct holes. Z31 easier to figure out due to layout of the dizzy head. You can use the plug wrench in the tool kit but this makes it easier.
Gear Wrench 5/8" x 6" Magnetic Swivel Spark Plug Socket 80546: Advance Auto Parts

pull the screws on the throttle mechanism where the cables run. Not absolutely necessary but makes things easier to get at #6

put a gage on the fuel system to be sure that is working correctly. put a vacuum gage on the system to see what that is doing.

download the fsm from xenonzcar. Read up on what is supposed to be happening not just pulling ideas from the air.
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