What can I do if the diagnostic scan doesn't reveal a misfiring problem? on 2000 Mazda Protege

My car is actually a Mazda Lantis, which is similar to the Protege. The problem is very sporadic and has been occuring for just over a year. One minute I'm driving and the engine is fine, then next it starts shaking, sounds very rough and loses power and/or is extremely slow to accelerate. In fact you have to depress the accelerator further down than normal in order to get it to move and it's even worse going up hill; it barely moves. There's a strong gas smell and I've been told it's noticeable to vehicles behind me as well. As I mentioned, it's very sporadic. When it's misfiring, it can last 5 seconds, 5 minutes or even for the entire drive. The misfiring can go in and out and there doesn't seem to be any particular cause; it happens when it's idling in traffic, going up a hill, driving on flat road. Some times I get out of the car to go to the grocery store or drop off my children, then get back in minutes later and it misfires on starting. Weeks can go by without any problems where the engine is running nicely and then it happens again. Or, as it usually the case, it occurs at such dangerous times as just before you turn into incoming traffic. I've learnt not to take any chances when turning across vehicles or using roundabouts as you never know when it will start acting up. We've had two diagnostic scans which didn't reveal any codes and I've been informed it could be anything from ignition coils to sensors. I'm extremely frustrated and want to know how to realistically diagnose the problem without randomly (and expensively) changing parts. Thank you for any help you can provide me with.

by in Bena, VA on September 07, 2012
2 answers 10 comments
ANSWER by on September 07, 2012
Check the engine compression, specification is 199 PSI. If the compression is low check for the timing belt to be off a tooth or for possible sticking piston rings.
COMMENT by on September 07, 2012
Thanks for your help! I'll have someone look at it
ANSWER by on September 07, 2012
WHO SCANNED FOR CODES? A MECHANIC OR AN AUTO PARTS PLACE?
COMMENT by on September 07, 2012
In both cases, mechanics. Why, does it make a difference?
COMMENT by on September 07, 2012
Yes. big difference in reading codes off a scanner and actually using the info to DIAGNOSE. OBD2 if your truly experiencing an ignition misfire , as opposed to an intermittent fault in another system that exhibits similar symptoms , then it should be seen on the live data under 'misfire counter' or something similar.
COMMENT by on September 07, 2012
Thank's) As for your trouble with the engine, has a MECHANIC had a chance to look at your car when it was acting up? Better yet with a GOOD scanner hooked up at that time? There are a lot of code readers that aren't designed to troubleshoot! However, some intermittent problems are verry hard to find. Hang in there one of your guys will find it! Good luck.
COMMENT by on September 09, 2012
And I don't think a compression problem , timing belt off a tooth , or sticking rings are problems that would fix themselves on and off for almost a year. DON'T forget the BASICS and forget the scan tool for a few minutes and use a vacuum guage. EYES are used on spark plugs (possibly with some magnification) to reveal other existing problems. Any history of body work would warrant an inspection of all grounds. ALL just random thoughts from an idiot. Hope they help.
COMMENT by on September 10, 2012
To wetry: The mechanics have asked us to bring it to them when it acts up, course the problem is you can never tell. Once we were on the way to them and it cleared up just before getting there. But thanks again for your response, I'll go back to the dealers and ask them to tell me EXACTLY what comes up, or as ziptie suggests get back to the basics. To ziptie12: Hey, random thoughts from an "idiot" just may solve this thing, so here's a big thanks from this idiot :)
COMMENT by on September 20, 2012
PLEASE let us know what the progress is , if any yet.
COMMENT by on September 25, 2012
None yet, since I need to raise the dough to go to get it checked. Sigh. It's been a truckload of money problems recently :(
COMMENT by on October 03, 2012
Been there , not fun. Hang in there.
COMMENT by on October 10, 2012
UPDATE: Finally! It's been solved and you'll never believe what the problem was. There was a reason nothing showed up on the scan; it was just that a connection was loose. Literally. My boyfriend decided to take the bull by the horns and replace parts piece by piece until the car was fixed. Since we've been getting advice from just about every body under the sun, we started with the coil pack. But after changing it (he did it himself after being shown how easy it was) the engine was still misfiring. It was only on checking the cables (if that's what they're called) that are at the end of the coil pack and fit into the engine/near the spark plugs that he realized one of them was actually quite slack and sliding off slightly instead of fitting flush like the other. After experimenting, he saw the difference in engine idling with the cable loose and then with it fitted. So, a little tape later, voila, the problem is solved (albeit spending unnecessarily to buy a new coil pack since the old one was perfectly fine). But I'm just relieved it's finally over, and a little weirded out that one tiny thing could wreak so much havoc! Thanks anyhow guys. At least if anyone else has a similar problem, they can learn from my experience and always check the little things first before assuming it's a really big thing :) Seriously though, could you imagine spending a truckload of money and the only thing needed was a piece of tape? I'm still shaking my head

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