Big backfire - why?? (long)

[racemor]

Ok, here's the story. I'm going to tell it in the order things happened. Kind of long but want to get all the details.

As some of you may know I had my car repainted so it was off the road most of the summer. Upon getting the car back in late September I put about 1250 miles on the car before returning it to the body shop for some touch up work in early November. The car ran great and looked good too! Just before returning the car my tach started cutting out. It would sit at zero for a few minutes, then bounce wildly, then be acurate for a while. Back and forth like this. I thought maybe there was a wire that was knocked loose when the dash was replaced. (I still have not check this for sure) Also, the hand throttle has come out of adjustment and I have to keep it pulled up about 1/2 way to keep the car idling (sp?) above 600 rpm.

This brings us to this past Saturday. I picked up the car. Started up fine and ran well but still had the idle and tach problems. Sunday afternoon I noticed it cut out as I am driving for about 2 seconds. Felt like it was running out of fuel but had 3/4 tank. Monday driving to work after about 20 minutes, car is good and warm, cuts out again and give a huge backfire. Then twice again this morning and multiple times on the way home this evening during my 35 minute commute. Seems to happen at about 3400 rpm and part throttle each time and the warmer the car is the more likely the backfire is to happen.

I have included the tach problem because I have heard that bouncing tachometers can be related to rotor problems but that may not be an issue here. There is also a strong fuel smell that started with filling the tank after getting home Saturday evening, no leaks that I can find though. But this has been a common problem with this car for a couple of years and seems to come from the charcoal canister or the connecting sleeve between the tank and the filler neck at full tank. Usually the smell goes away at 1/2 tank but not this time.

A little more background. This is a stock 2.4 with mfi, ssi, dual out muffler. I had previously had a flat spot/bogging feel that went away with a good mfi linkage adjustment in Spring of 2002. That problem always seemed worse in colder weather. I don't think this is the same since the problem is much more severe now and weather was in the mid 50s here for the past few days.

Any ideas? Timing? MFI adjustment? Bad gas? I'm going to have my mechanic take a look as soon as I can get the car there, but it is always good to have input from you guys.

Thanks in advance and sorry so long winded.

[Superman]

I heard of a guy who, when he was a younger man, he and his friends would have fun turning the ignition off and then back on, coming down hills. Makes big banging noises. He eventually grew up to be a productive citizen.

Your ignition system is turning off, and then on again.

[RoninLB]

well Tom... I can't confirm your problem, but had the same thing happen without the backfire.. was ignition/replaced CD and coil.. and then another time. KaBoom. that was a loose dizzy electric wire..........Ron

[racemor]

Super, could that "guy" have grown up to drive an '83 SC?

A loose wire would be great if it is that simple. I'll check it out.

Thanks for the help! I'm not to good at diagnosis, obviously, but trying to learn as I go.

[Wayne 962]

The MFI system has many compensators in it that try to adjust the mixture and prevent backfires. Is your heater hose properly hooked up to your MFI pump?

-Wayne

[cegerer]

"I heard of a guy who, when he was a younger man, he and his friends would have fun turning the ignition off and then back on, coming down hills."

LOL! A friend and I blew the exhaust system off of his dad's new truck - twice - doing backfires while cruising by the crowded bus stop in front of our school ....

Tom, hopefully it's something that simple! -- Curt

[Bill Douglas]

Yeah, a friend of a friend does that trick with the ignition off. He's 43 now, I just wish he would grow up...

[Superman]

Why yes, he did eventually drive an '83 SC.

The kinds of wires you would check first are the ones that provide power to the ignition system generally. So, the ignition switch comes to mind for example. The aforementioned troublemaker would probably get the car idling, and then wiggle the key, wiggle the back of the ignition switch carefully, wiggle the wires coming off it carefully. He'd probably do the same thing to the wire that feeds the coil and the CD box and stuff like that. He'd listen for idling problems at that time. This is not just a deal where maybe a spark plug is not firing occasionally. It's more likely a situation where the entire ignition system is shutting off briefly, filling the exhaust system with fuel and air. At 3000 rpm, the engine makes fifty revolutions per second.

I know nothing of MFI and Wayne does, but I'm not sure why any decent fuel system would have backfire trouble, unless it was making a mixture that is too rich to burn in the combustion chamber. In my experience, if the mixture is even close, and the spark is hot and timely, then the explosion happens in the combustion chamber. I'm sure Wayne is right, I just don't understand that system.

[nostatic]

Originally posted by Superman
He eventually grew up to be a productive citizen.

can a flaming liberal pinko really be considered a "productive" citizen?

{ducking a thrown head stud}

hey, can I borrow this?

[Superman]

I know you, Doc. Thanks for the compliments.

[yelcab1]

I would check for bad connections of ground wires, sensor wires, to all the ignition components. Since it is an intermittent thing, it points to something like that.

There are only so many wires in the ignition system, if you are methodical, you'll find it. Also, make sure the connectors are clean bright and rust free /crud free too.

[racemor]

Wayne - having gone through trying to figure out the previous mfi misfire the heater hose is the first thing I checked. Looks fine.

Mr. Man (or may I call you Super?), since you mentioned the ignition switch it makes me wonder if there could be a problem with something in the dash. Is there a ground wire or something I should check that could also be effecting the tach? I mentioned that the dash top was replaced while the windshield was out for the paintjob. If so, maybe it was knocked loose at that time and is now getting worse. Or am I way off base?

[Steve W]

Backfires are often the result of a fuel mixture or delivery problem, or perhaps a intermittent ignition from something like leaking plug wires. You either are running too lean at the load point when the backfire occurs, or you have an air leak somewhere causing the lean condition. I would venture to guess you have a large air leak somewhere in the intake system. When the mixture is too lean, the fuel mixture does not fire, so you end up with raw fuel in the exhaust. On the next couple of cycles, the combination of excess unburned fuel, and with the newly injected fuel is finally rich enough to be ignited, along with the excess fuel in the exhaust, giving you that big backfire coming out of your tailpipe. If you were to monitor the exhaust mixture, you may notice what may be confusing at first, an excessively lean mixture, and with a light backfire, what is an excessively rich mixture for a split second. Does my explanation make sense?

[john walker's workshop]

sounds like the points are barely opening, or burnt or dirty. tach bouncing, cutting out, backfire, etc. all points related. could it be so simple?

[jkeyzer]

I totally agree with JW. I have had all these problems - points were the cure, in my case, an electronic replacement put my mind at ease.

[beamonk]

I had a humongous backfire once running down the highway, distributor cap had only one arm holding it on. scared me to death, totally unexpected. Usually from inginition problems.

[Superman]

One time I ignored John Walker's advice. Another time I just was not listening closely enough. both were mistakes. I'd check those points first.

Respectfully, I'd say the dash theory is a crap shoot. You probably can't troubleshoot this from your computer. If it is a bad connection, you need to wiggle some wires. but as smart as I think I am, if JW says everything points to the points, then check the points next. We can't troubleshoot these things from a computer, but JW can.

[racemor]

Thanks for all the help guys. I'll check the points first for sure.

[Targa77]

My car did this once and it was the points. They weren't opening up enough.

[bigchillcar]

As soon as I read the beginning of your thread I was thinking points. Mine closed up on me last year...same symptoms and it happened again around Thanksgiving on the highway. I had to pull over and re-open them...replaced them as soon as I got home. That's your problem as well you'll find.
Ryan