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So i have a 1989 VW cabriolet that i bought 6 months ago and did a basic tune up because it ran so terribly.
Did cap rotor and wires with in this year.
Oil change fuel filter etc.
Ran ok but had stunbling issues.
Guy who was a CIS "expert" tuned some things.
Passed emissions and went ok but needed CO adjustment but couldnt find a long allen t at the time.
Got that running nearly perfect and it picked up amazing for half a day (wasnt running lean anymore)
All the sudden it misfired for a couple seconds after restarting so figured was rich (yes i know i should be adjusting co in accordinance through test port and not ear) so turned it counterclickwise a 1/4 turn and misfire tapered.
Then ran good till got home and idled for 5 minutes and nor have a constant misfire like when i got water in my mk3 cap driving in a puddle.
Gone through it all and cant figure it out.
Thought i messed up my distributor adjusting it so through a refurb with no luck albeit does run better with most all parts new and no vacuum leaks now, but still a misfire.
Down to 2 last things to test or try and thats the coil or coil pickup.
My question is whats the likelihood that my coil is not completely failed and passes the tests but have a misfire? Same with the coil pick up in the distributor.
Oh and replaced a bad fuel injection relay since it was badly corroded... idk how the car ran with out it? And literally with it unplugged so am wondering if one of my fuel pumps are bad. Also havent dropped the tank yet.
Im going insane here and luckily passed emissions before this and registered it
 

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Does the engine misfire when it's cold? Hot? Long periods of idle?

CIS is a very easy and reliable system but the problem is that people just start throwing parts at it and make adjustments without any clue to what they are doing. Once this occurs, they get PO and complain about the system.

I would start with reading up on how the system works. Know it like the back of your hand. Then buy a good fuel pressure gauge and adapters to hook it up. I would also understand how the lambda system effects the system and adjust the idle mixture with a dwell meter. Set up the system so the pressure are correct [hot and cold].

Once the system is set up properly, leave it alone. Forever! It's that good.

As for vacuum leaks, how do you know you do not have any? Only a smoke tester will let you know if you have none. CIS system are very picky about leaks. All the air must go thru the air flow housing or else the fuel will not be delivered in the right amount. This is by far the biggest issue that people do not deal with head on.

Fuel injectors can be an issue but it's hard to test without a good pressure tester. I always believe it they are several years old, it might just be better to replace them. If the fuel injector does not spray a fine mist, it will cause the engine to misfire. Liquid fuel does not burn so that is why the large droplets of a bad injector will misfire. I hear Mercedes injectors are the ticket. They are cheaper. Fuel injectors do not control flow, they just spray fuel. The fuel distributor controls the flow. There is no such thing as a high flow CIS injector. Some may flow more than others but they do not control the amount of delivery into an intake port.

If the in tank pump is faulty, it can cause the system to vapor lock. Vapor in a CIS system is no good since it's a hydraulic system [kinda like air in a brake system]. This only occurs on hot days with a long idle [stop and go traffic too]. As easy as it is to replace the intake pump, if it has over 100k, I would replace it.

The main fuel pump can be noisy. The noise is either they are worn out or there is vapor in the system. A cavitating fuel pump is noisy and that is the main reason they are noisy [of course a 200k fuel pump is worth replacing because it's just old].
 

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what type of cap and rotor............

so i have had a misfire issue as well for sometime, recently i talked with Joey at doubles J's motorwerks in Portland OR. he advised me about how there are mexico/Chinese made Bosch and german made bosch.....the german Bosch rotor will have a much wider contact area at the end of it. where the cheap one is a short "T". i went to Tectonics tunings website here's what i bought. fixed it right up, along with some tuning adjustments.

Total Items: 3 Weight: 2.5lbs Amount: $92.00

Distributor Rotor ('85-early'99 8v and 16v)
$10.00

Distributor Cap ('85 and '92 MK1-Mk2) 4 cyl.
$22.00

TT Blue Spark Plug Wire Set 1985-1992 Mk1 Mk2 8V
$60.00
 

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I have had a stumbling issue before that went away once getting up to say 20-25mph. Turned out to be a cracked vacuum hose. Pinch, squeeze or pull on each hose. Sometimes tricky to find.
 
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