M20 only firing on 3 cylinders

[Kevinst609]

After lurking for 2 years this will be my first time not being able to solve a problem with a post on this site. 89 m20 motor started running rough out of the blue. Sounded like it was only running on 5 cylinders and got progressively worse like it was losing spark on additional cylinders I had a similar problem which was cause by water in the afm. Lead me to believe something similar happened again. A new afm didn't fix the problem.

Which made me think the ignition might be going bad. I've replaced the distributor cap as well as plugs and wires shortly after purchase. About two years ago. Distributor within last 12 months. I noticed that I wasn't getting spark from 3 of the distributor outlets before the wires with a simple screwdriver test. I thought maybes installation may have given it a shortened life. But wouldn't that have happened very shortly after?

I recently threw an O2 sensor code which I plan on replacing. The main problem needs to be fixed beforehand. Again first time posting on this site since all my other problems were fixed with other threads. But not his one. Thanks guys!

[george graves]

Welcome aboard!

Which made me think the ignition might be going bad. I've replaced the distributor cap as well as plugs and wires shortly after purchase.

Did you replace the rotor? As far as ignition "going bad" it's all in the ECU and coil really.

I'd re-do your spark test on all the cylinder, if it was my car. I'd also pull the plugs and put them in order and post pics. (plugs can tell alot about whats going on)

What's the exhaust smell like? If you're missing spark on even one cylinder after warm up it should reak of unburnt fuel. with 3 it should smell like a gas pump.

If it's not spark, I would think it's a bad injector connector. Ask me how I know....

[Ether-D]

Did you replace the rotor? As far as ignition "going bad" it's all in the ECU and coil really.

I'd re-do your spark test on all the cylinder, if it was my car. I'd also pull the plugs and put them in order and post pics. (plugs can tell alot about whats going on)

What's the exhaust smell like? If you're missing spark on even one cylinder after warm up it should reak of unburnt fuel. with 3 it should smell like a gas pump.

If it's not spark, I would think it's a bad injector connector. Ask me how I know....

I like this. The injectors fire in groups of three, right? Or do I not understand "batch fire"?

[IceWhite]

After lurking for 2 years this will be my first time not being able to solve a problem with a post on this site. 89 m20 motor started running rough out of the blue. Sounded like it was only running on 5 cylinders and got progressively worse like it was losing spark on additional cylinders I had a similar problem which was cause by water in the afm. Lead me to believe something similar happened again. A new afm didn't fix the problem.

Which made me think the ignition might be going bad. I've replaced the distributor cap as well as plugs and wires shortly after purchase. About two years ago. Distributor within last 12 months. I noticed that I wasn't getting spark from 3 of the distributor outlets before the wires with a simple screwdriver test. I thought maybes installation may have given it a shortened life. But wouldn't that have happened very shortly after?

I recently threw an O2 sensor code which I plan on replacing. The main problem needs to be fixed beforehand. Again first time posting on this site since all my other problems were fixed with other threads. But not his one. Thanks guys!

Ether-D, he says ... (As outlined in the quote)

[jlevie]

I am inclined to suspect one or more cylinders isn't getting spark or fuel. You stated that the distributor and ignition wires (and presumably plugs) were replaced two years ago. But I don't know what parts you used. Only OE or OEM parts are trustworthy. I'd also suggest a fresh set of NGK ZGR5A plugs.

Then tee in a pressure gauge to the fuel rail supply line and see what pressure you have while cranking. It should be 3.0bar or a little less if the throttle is closed. If the pressure is okay, get a noid light (they are cheap) and check to see if a firing signal is reaching the injectors while cranking. It isn't strictly necessary to test each injector. The DME fires the injectors by banks with bank 1 being cylinders 1,3,5 and bank 2 being cylinders 2,4,6. So checking one cylinder from each bank is usually sufficient to determine whether the DME is trying to fire the injectors. As an FYI power is supplied to the injectors when the main relay closes (which powers up the DME) and the DME grounds each bank to fire the injectors.

Based on the reported history, I doubt that there is a problem with the cam or timing belt. But to be safe pull the covers and check cam timing and belt condition. I have seen an old belt jump a couple of teeth and cause problems similar to what you described.

[Kevinst609]

I replaced the cap and rotor at the same time. I can't recall if I used oem or not. I haven't checked since my car is in another state. I have yet to mess with the injectors and rail. But what would cause the distributor to only give spark to 3 of the six wire connections?

[jlevie]

A bad distributor cap can do that as can an improperly installed rotor.

[afelice]

I had the same issue but I lost spark on 2 cylinders before it got much worse. When you take the rotor off again as I assume you will check for black residue that looks like lightning this will show if there is a misfire in the rotor. I'd also pull the rocker cover and check your rocker arms if you broke several arms this could also cause your symptoms


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