Backfiring through intake
 

Hi folks. I need some help from someone who's a lot more experienced than I am.

I was de-winterizing the engines (2000 Mercruiser 8.2L (502CI)) MPI and the starboard engine is giving me a fit. It ran perfect until when I just tried to start it this week. When turning over, it will idle but the idle is erratic and it will backfire through the intake. I did a compression test (all were within spec), checked the spark plug wires a half dozen times, made sure the wires are routed right, installed new distributor cap, installed new ignition coil.... .but still backfiring through the intake. Plugs looked fine as well.

What should I do next? Could it be a bad / clogged up fuel injector?

Any help or suggestions would be greatly appreciated.

I'M NOT A MECHANIC

Could it be the timing? Bump the distributor and knock it out? Lord knows I did that by accident in a car I once owned - drove me crazy.

Just a thought

3pointstar

Haven't bumped the distributor. It's in the same place bolted in just the way it was when I put the boat away for the winter. I have to loosen up bolts to move it.

intake valve may have some rust build up,,it's not seating properly,just back firing thru to the intake.

Wouldn't the compression be low if it was a valve issue?

a bad injector would be a dead miss. no fuel to backfire with. yes, a bad intake valve, bad enough to blow back like that would show almost no compression. do both these engines turn the same way? or is this a shaft boat, not I/O? check plug wires again. it is always plug wires or some other spark going to wrong plug thing.

Did you pull distributor to ck gear and shine down in and take a look at the cam gear. Also double ck ignition grounds. All grounds for that matter. Your symptoms sound almost like your dist is 180 off which I know isn't the case but. Ignition module?

It sounds like an ignition problem. Check all plug connections to ignition system for corrosion. Pick up module in distributor could be a possible cause as well.

You said you changed dist cap--what about the rotor???

Checked plugs and firing order several times. Did not replace the rotor because I really don't want to just throw parts at this.

Ya be careful those rotors are pricey. Lol. Should replace at time of cap as a rule of thumb. Kinda doubt it's the problem though. Isn't this fun.

Bad plug can and will do it.
Bad distributor pick up can do it.
Weak or water in fuel can do it.
As Sutphen30 said, valve not seating can definitely do it. So could a broken valvespring.

As with all driveability issues, basics must be done/checked first. People's minds like to scatter and lose focus before this happens.

Intake valve hanging up intermittently at idle. Does it miss if you bring up the rpm's?

Does it backfire steady with every engine revolution when running,,or just an occasional backfire.

It misses with engine rev.

A clogged injector will in fact cause that, had one that acted the same way, popping...

If ir backfires thru the intake with every revolution while running you have an ,exhaust valve Not opening,, pull the valve covers ,,check if a rocker arm stud broke or pushrod came off,,, I broke an exhaust rocker arm stud years ago, it popped thru the carb. with every revolution when running,, compression will check out ok because the valve is closed.

Yanking the heads asap. Will let you know what I find out. Gonna go ahead and replace the manifold & riser too while apart.

add in collapsed lifter from sitting up over winter valve compressed
or some ex valves got rusted stems and sticking as stated

Turns out I had bad manifold that leaked over the winter. When I started it up, had stuck valve which caused bent pushrod and a few other bad things to happen.

I would take a shot at freeing up the valve and doing a compression test.

Pushrod is bent. couple of valves shot. sending to machine shop to have work done and heads cleaned. Also going to go ahead and get new manifolds & risers for both engines. Turns out one of the manifolds was leaking which is the root cause of all this. Live and learn.

Salt water ?

Which manifold? Gil style? I believe the risers, being stainless, have very long lives. The manifolds are the culprit.

=Blown284;4554613]Pushrod is bent. couple of valves shot. sending to machine shop to have work done and heads cleaned. Also going to go ahead and get new manifolds & risers for both engines. Turns out one of the manifolds was leaking which is the root cause of all this. Live and learn.[/QUOTE]

Good idea, don't patch it up,,send to the machine shop and get it fixed properly .,save a lot of problems later on.

Yep.

Stock mercruiser style... not stainless.

Ok, then look on ebay for stainless risers that fit stock manifolds.http://m.ebay.com/itm/816900A7-Stain...%257Ciid%253A1 you can usually find en about 150 a pair.