Backfire

[ralphcap]

So I hop into my ''81 2000 FI for a nice weather spin. When I turned the ignition key there was a loud backfire. Then, I was unable to start it after several attempts. Not a good thing eh? What is the likely diagnosis?

[RoyBatty]

I'm not an injection guy, but I think you're about to hear the guys that know, tell you about the backfire wedging the airflow door in the injection air metering system.

[So Cal Mark]

it may have blown the intake hose off of the air flow meter

[GeorgeT]

It's possible that the Air Flow Meter flap warped from the backfire and now doesn't freely swing. I would start there.

[majicwrench]

I would start by seeing it you have spark, then see if you have fuel. Could be anything, ya gotta look.

[Kevin1]

The same thing happened to our '80 fi. The plastic air hose blew off the air flow meter. Nothing was damaged, reattaching the hose fixed it.

[bradartigue]

Typically caused by a sticking intake valve. We had an 81 that did this religiously, especially on cold days. It will damage the AFM if the hose doesn't blow off first. It was violent enough to blow the little safety door (on some AFMs there is a spring-loaded circular door in the middle of the flap) to pieces. We didn't bother to diagnose it to which valve, once the head was off we replaced all of them. Not necessarily caused by a bent valve, though a slight bend might do it, just more from crud (I think) from years of use.

[majicwrench]

With all due respect, let's look at some simple things first. After almost 40 years of fixing cars, I can count on one hand the number or sticking valves I have seen.

[ralphcap]

Thanks to all. I'll be looking into the problem on Monday (hopefully). Currently over 1400 miles away in Colorado.

[bradartigue]

With all due respect, let's look at some simple things first. After almost 40 years of fixing cars, I can count on one hand the number or sticking valves I have seen.

Five then?

Of course it's not a final diagnosis - how the hell would I know what the guy heard? I can assure you that air doesn't flow backwards in a motor unless something is out of whack.

[So Cal Mark]

and without spark, no backfire

[majicwrench]

OP said "unable to start it after several attempts" he needs to check spark and fuel.
Last thing he needs to do is pull head and check for sticking valves.Let's get real.

[So Cal Mark]

no spark, no backfire. Check the intake hose from the Air Flow Meter as a first step

[bradartigue]

OP said "unable to start it after several attempts" he needs to check spark and fuel.
Last thing he needs to do is pull head and check for sticking valves.Let's get real.

It is reasonable to assume, given the description, that the AFM door was blown backwards. I think the most sane approach, before tearing apart the ignition system or unhooking injection lines, is to see if the AFM door is intact and swinging. If it is then move on. If it is not then something blew it backwards - and I'm guessing it was air - which blew out of the motor when an intake valve stayed open a moment too long, which I've seen happen on a couple of Spiders including our 1981. You can remedy this with an oil change and a little of that CDC valve cleaner, though I would run it through and change oil again in 500 miles. If it happens again then check the head.

[majicwrench]

Yup, engine wont start, but lets's just assume he has spark. Obviously, he HAD spark once upon a time, but how about we check now that car wont run?? Certainly check hose first!

Sure, it's POSSIBLE that a valve hung up, but realisticly, no.

Let us know how it turns out.