How do you test ignition leads?

Some ignition wires have resistance measurments on them but honestly the best way is to replace them. If you only have one suspect you can try swapping it with another wire to see if the problem moves cylinders.

Never heard of testing a coil with an ammeter before. There should be resistance measurement for the coil but even that is hit or miss. A coil can pass all the tests but fails when put under load.

Craig

One test that often helps diagnose problems is to look at the engine in the dark in the presence of heavy fog. Lacking the timely heavy fog, spray a very fine mist of water over the ignition system in the dark (after your eyes adjust to the dark). Let any arcing show the need for action.

No arcing does not indicate the lack of a problem, but arcing indicates the presence of a problem, and it shows the location.

Looking at the spark plug electrodes can show a problem associated with a cylinder. A significant difference would indicate a problem. But once you have the plugs out for inspection, it seems like a good time to replace the plugs if they have not been changed recently.

To directly answer your question, you could stick a wire into one spark plug wire that has been unplugged from the spark plug. See how big of a gap you can make the wire jump to the block from that added wire as the engine runs on three cylinders. I would expect the gap that it would jump to be maybe 1/4 inch (much bigger than a spark plug gap). What would take a lot of description would be to explain how to not get shocked by the 50000 volts or so. You need a lot more insulation than you might imagine when dealing with high voltages. Some others may have some suggestions for doing a test that is less dangerous than the one I might do with my own car.

Thanks Craig. Yes you're right and I did wonder if that was good enough. All it proves is that there is a DC path through the coil but I've never had a coil fail before ever. If all else fails I'll change that too. I cant swap the HT leads around as there are hard wired into the cap but they are as old as the car (1985) and may need changing anyway. Just trying to keep costs down. I've already spent more than the car is worth! The car overheated big time a while back. It drained completely of water and I drove on about a mile like that not knowing. After that I had the head skimmed and reassembled the engine with the result that it ran 'lumpy' which you know about. That was cured by retarding the ignition following your advice. But it still isnt 100% smooth which I'm pretty sure is electrical by the sound of the engine. The rotor arm had gone open circuit between the centre and the tip which was probably due to the overheating. It should have had a resistance of 1200ohms but was open. I've changed the plugs already and it's still the same. Oh well more money!

Cheers Roger

Craig wrote:

Hi Tom. Yes I've changed the plugs and checked them after running the engine. They all appear the same. Dont know that I'd risk the water treatment!!! but it's a novel idea if all else fails. But the wire against the block sounds like a good test. I'll give that a shot. Many thanks

Roger

Tom's VR6 wrote:

Hmmm Did you do a compression test AFTER having the cylinder head reworked? I will ASSuME that you have hydraulic lifters and they are quiet. I might be one of the few that still have solid lifters in my engine which need periodic adjustments with shims. I have seen one vehicle that had so much oil pressure that the hydraulic lifters pumped up and kept the valves open causing the engine to stall.

Other causes could be a faulty fuel injector or even it's seal. Check the seal by spraying a little carb cleaner at it while the engine is running noting any idle speed changes. Vacuum leak or faulty component causing a vacuum leak near the cylinder that is not firing well.

Thanks Dave but I dont have hydraulic lifters by which I assume you mean valve lifters. I have an overhead cam that operates the valves directly via shims. When I had the head skimmed I ground in the valves and changed the shims for thinner ones. I've checked and rechecked the valve clearences. Thay are all within limits although one is exactly ON the limit. But I dont think that would cause a problem. I've also checked the compression using a tester and that is within limits too. I've used a vacuum tester on the vacuum pipes and checked the vacuum advance on the distriburor with it. Unless I'm doing something wrong they all check out. The car has a carburettor and apart from removing and replacing it I did nothing to it. But you have a point about the seal between the carb and manifold. It's as old as the car but if it was leaking wouldent that affect the firing of all cylinders? It's perplexing to say the least. Cheers Dave

Roger

dave AKA vwdoc1 wrote:

Hello Jim. By the sound.

To cut a long story short after the engine overheated (I ran without water for a mile or so without knowing) I decided to have the head skimmed and at the same time I ground in the valves. After putting it all back together it ran 'lumpy'. I mean really just about firing and threatening to stall all the time. It was so bad the car shook. After much poking and hoping I noticed the engine timing marks on the pully and intermediate camshaft gear would not line up. This was because reducing the thickness of the head (the head was not reduced beyond the limits specified in the manual) it effectively shortened the distance between the main camshaft gear and intermediate camshaft gear and since they are connected by a toothed belt lining the marks up was not possible. Craig then pointed out that to make up for the marks not lining up the ignition timing could be adjusted. So I retarded the ignition while the engine was running and behold the shaking stopped. But it still doesnt run 100% smooth, or as it ran before, The engine sound is exactly the same as with a misfiring plug. It runs smoothly uneven if you know what I mean. The engine responds to the throttle immediately, as it should, but so does the uneveness. Apart from more poking and hoping I dont know what else to do. Thanks for your interest.

Roger

Jim Behn>I have lost track of the thread. How do you know one cylinder is not

From: "ukdodger via CarKB.com" snip

OK I think I understand more of the facts....... Overheated engine.......cyl head warped......head resurfaced and valves reground. Carburetor Engine with shims to adjust the valves. Adjustment OK Compression OK Not running on all cylinders at idle and during acceleration.

Is there an EGR system on this engine? I am still looking for a vacuum leak somewhere. ;-) Do you know which cylinder is not "pulling its weight"? IF the brake booster is leaking that could cause a leak near one of the cylinders creating that miss. Block all large vacuum ports temporarily. Did you time the ignition with a timing light? I will ask a stupid question...............do you have the firing order correct? I think you mentioned that all spark plugs look fine and they all get good spark at the correct time. I have not seen spark plug wires that were permanently attached to the distributor cap, but I have had several sets of new Bosch spark plug wires that were not good.

It sounds like you did some of the work yourself. Did you make sure that the valves, guides and springs were fine? Did you install new valve stem seals? Did you remove the intake manifold from the cylinder head. I assume that you did. It could be a problem with the gasket. Maybe some of the old gasket still existed while you installed the new intake gasket. Again spray some carb cleaner around the intake/head gasket area and see if you notice an idle change. I have seen some intakes that were not fully torqued down and that created some running and idling issues. Vacuum leaks will cause some cylinders to run leaner than others and might caused your 'dead miss'.

If you truly had the head shaved that much you could get an adjustable camshaft gear. The way I test wires is fairly simple. First test on a humid night or spray water from a spray bottle looking for leaks. Test two: Attach a timing light close to the spark plug to see if juice is getting that far. Test three: Stick a 1/4" in each spark plug wire and hold a half inch from the block. Start car and see if the spark jumps the gap.

You waste a lot of time trying to get the car to run right if the camshaft is retarded too much. I know when I changed timing belts on the 1984 and older VWs I could get a decent idle but as soon as I drove around the block I knew the cam timing was off. I had some welding done on my Toyota truck head. I had the head shaved after the welding, guides knurled or replaced as needed and new stem seals. Truck runs fine even with the cam slightly retarded. I would imagine on the VW that a shaved head is going to be a lot less camshaft timing error than being one tooth off.

I am not sure what bad wires sound like. I have maintained my cars for the last 31 years. I don't know that I ever had any wires get that bad that I might hear anything besides an occasional skip (misfire). But I don't drive much. I might have a million miles on the various cars I have owned. OK, the math does not work out well. I guess closer to

900,000 miles.

You personally ground the valves? You did not have the valve guides and seals replaced?

So you are stating that you have used a timing light to check the timing on the flywheel? Anything less is just guessing. While you have the timing light out you can check those wires for rudimentary function.

What are the read>Hello Jim. By the sound.

The camshaft timing is perfectly fine. It was his intermediate shaft marks that wouldn't line up.

If it truely is an ignition misfire you should be able to pull each spark plug wire off the engien one at a time. The one that effects the engine the least when removed is most likely the cylinder with the misfire.

Craig

So, we're talking about a distributor cap and ignition wires that are 24 years old? Just replace them!

Hi Jim, Craig and Dave.

Well I did the spark plug test by jumping the spark. All Ok. About a 1/4" as you said. Then as Dave mentioned I checked the vacumm hoses and discovered one hanging loose and going nowhere. All it did was operate a switch to change gear but it meant somewhere was a leak in the vacuum system. After a lot of swearing trying to find where it went I eventually removed the carburettor and found the connecter tucked up underneath. Ok so I reconnected it and started up. No more shaking and no misfires. All appears to be as before.. but but but..one more problem persists. When I rev the engine up and take my foot off the throttle the revs dont immediately fall. It's a second or more late. But when accelerating the response IS immediate. I'm no expert on carburettors so can you throw some light on what may be wrong. It's Pierburg 2E2 and has only one adjusting screw which adjusts the tickover mixture. I adjusted it with the engine hot for maximum revs but it made no difference to the engine decelleration. I checked the throttle cable and linkage. Both move freely. Thanks for your help so far guys. I appreciate it.

Roger

Jim Behn>If you truly had the head shaved that much you could get an adjustable

There should be a little diaphram looking thing that the throttle linkage hits to allow the throttle to close slowly & not stall out the engine. Make sure its moving freely.

Cool.

Does that engine have a dashpot? Maybe it is adjusted too high.

Thats the word I was looking for LOL. Dashpot.

Craig

lol This feels like an Olympic event! ;-)

Glad you are getting the problems solved one at a time. You are gaining a lot of experience now as you go for the GOLD!

If your carb is not equipped with a dashpot, or other device that slows the throttle plate returning to rest, then you might have another vacuum leak along with maybe a rich running carb. The vacuum leak could be a vacuum reservoir located inside of the fenders in some cases. OR it could be at the base of the carb with maybe a defective gasket. OR maybe a mixture adjustment to lean out the carb a little.

FYI picture of carb here

#40 is a device that holds the throttle plate from resting momentarily#10 should be your idle mixture screw. read here too

One challenge is US based VW fans may have never seen the carb you speak of. I have seen 2 Carter carbs on 83-84 models. Everything else I have touched has fuel injection. I think 1984 was the last year for carbs in the US but I could be wrong. But I am just a VW fan, not a full time mechanic.

Number 40 might be an idle boost? Does a Quadrajet have any more parts?

Lubricate l>lol