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Possible causes of a shaking engine?

Hi everyone, this is my first time posting here, I have a 2001 Mazda B3000 that was running fine until yesterday. I had spun the truck around to empty water from the bed after washing it when it started to run rough, before I could diagnose anything it started shaking badly and gave me a light then a blinking light. I immediately shut it off and checked everything. Found one plug wire that had been touching and burned through, replaced that and it was still rough. Replaced the ignition coil and still rough. Now I find out I’m firing on all but the #1 cylinder which is still getting spark. Going to do a compression test tomorrow but not sure if it could be something other than a valve or head gasket. Any ideas? BTW, the truck has ran for less than 5 minutes from the time the light started blinking so I’m not sure how much damage could be done in that short of a time. Thank you!

You might try swapping the fuel injector from cylinder #1 to another cylinder to see if the miss moves with the injector.

If it does, that’s the problem.


I’m not the most vehicle savvy person, literally figuring everything out as I go with with car stuff. There’s wet gas on the spark plug when I pull it off while all the others are dry, would that rule out an injector?


The injector could be stuck open dumping gasoline into that cylinder.


Gotcha, will give it a go tomorrow then! Anything to avoid a $1600 repair on an $800 truck at this point. Thank you!

"I’m not the most vehicle savvy person, literally figuring everything out as I go"
You’re doing great so far. You’ve managed to trace the shaking to cylinder #1 and posit a good theory based on sound evidence.

You might want to go easy on the spinning the truck activities, however. 15 year old Mazda pickups will inevitably revolt against that type of treatment.

I’m fairly mechanically inclined, I just haven’t worked on very many vehicles before. I only turned it gently, try not to abuse it too much lol

It’s almost certainly among spark, fuel, or compression, so you seem on the right track. Is it possible the truck-washing job somehow got a high voltage ignition component in the engine compartment wet? You might just need to dry the engine compartment out if that’s the case. If your truck is configured with a single shared ignition coil and a conventional dizzy and dizzy cap, might be a good guess to just replace the dizzy cap, the wires, and ignition rotor. If that hasn’t been done in the last 30 K, it is probably due anyway. Good idea to check for a good healthy visual spark at the spark plug electrode of the suspect cylinder too, if that’s possible.

Running the engine w/a substantial miss can damage the cat, a big $$$ repair, so best to minimize engine running time until this is resolved.

New plug and coil wires all around might be worth a shot.

This engine has a multi coil pack, up and left of center in this image, which has already been replaced.

All’s left are the plug wires.
They can be tested for continuity with a meter, but that won’t tell the whole story; and with one already failed the others can’t be far behind.
Easier to change the rest as a first move before dealing with moving injectors.

Continuity checks won’t test for other problems, such as HV breakdown paths.

Still waiting to do the compression check and check the injectors once I get off of work, I replaced all of the wires when I replaced the one, sorry should have said that. I moved the plug from the bad cylinder over and it was firing fine over there, the plugs and the old wires have maybe 1-2k miles on them. All of the plugs seem to be getting a good spark, but I don’t have any readers to test everything. Running on the pay of a 23 year old construction worker so I’ll have to wait until Friday to buy or rent a code reader and electrical tester.


This is basically a Ford Ranger if I am not mistaken… I just recently repaired one of these so its fresh in my mind… Yes…the wires…we never suspect wires till they fail…and FAIL they do…in all sorts of strange ways.

I would put a new set of wires on at this point in time. The most common failure I see in these is on the passenger side…the wires touch the metal heat shielding around the AC portion of the firewall…this metal heat shield abrades the plug wire casing…and they leak voltage. In fact the last 3 of these I worked on needed new wires and a careful wire re-route to avoid the passenger side heat shields.

Of course this issue can present anywhere a wire touches anything it can ground out on…

I wouldn’t suspect anything else till I had a new set of wires on this engine… It worked for me the last 3 times…


Update: So I still haven’t gotten around to the compression or injector testing, just finished up my 2nd 12 hour day at work, but I’m thinking against a head gasket due to no oil in coolant and vice versa, and no smoke coming out of the exhaust.
I did however change the fuel filter about 1k miles ago to try to fix a problem with the engine bogging and skipping at anything above 70 mph (which it didn’t fix.). The previous owner had the wrong filter installed, long shot but would this have worn down anything critically enough to give me the problems I’m having now? Other recent repairs include replacing the thermostat, and bypassing the heater core, and replacing ball joints of course.

Nope. An incorrect fuel filter could cause bogging if it lacked sufficient flow when accelerating or on the highway use, but once corrected there should be no residual effects.

Why was the heater core bypassed and the T-stat replaced? What were the symptoms?

Random small amounts of overheating, it never buried in the red, just went up about 3/4 of the way once every driving session. It’s been fine ever since. As for the heater core, antifreeze all over a friends leg and foggy windows for days on a trip to Florida from Georgia not too long ago.
I didn’t have much hope for the filter being suspect, grasping right now hoping for something simple to save her, but not looking good lol

A wet spark plug can also be due to an ignition miss. Faulty spark plug, wire, coil terminal, etc.

Low compression can also cause a spark plug to appear wet. Don’t take a pessimistic view yet because of that comment.

I’m a firm believer in a compression test on any engine with a performance problem. Make sure the engine is mechanically sound before spending various amounts of money on it.

If you have an old-fashioned ignition timing light with an inductive pickup, you could use it to determine if the #1 spark plug is getting high voltage. If it doesn’t make a spark, there will be a buildup of liquid gas on the firing tip.

The last 3 of these I worked on …ALL needed new wires… per my info prior… Just reiterating…