Backfire in manifold

1998 classic, 4.0, AW4

So, while I was out wheeling a couple weeks ago, I got stuck in a mud hole and ended up overheating. The temp. gauge was pegged and the engine started to smoke, I'm guessing from burning off some oil, etc. I shut it down after I got pulled out as it was pinging really badly from the fuel pre-detonating due to the high heat and running like crap. I noticed that I blew the lower radiator hose and I opened up the radiator and it was bone dry. We were able to get it off the trail with some band-aid fixes and I had it towed home. I checked the oil and there wasn't any water/coolant in it.

I flushed the entire cooling system and replaced the hose and the temperature is fine. However, now there's a couple problems. Immediatley upon start-up it idles a little rough for about 10 sec.
Within the first 1-2 minutes of running after start-up (I'd say it does this 75% of the time), if I'm on the gas it will just bog down completely and then a loud "pop" backfire in the manifold and it's back to normal.
The third issue is that after running for prob. more then 10 min. the idle becomes a little rough again.

I thought maybe this was from my upstream o2 sensor, that maybe the wires had melted with the heat, as I know that will cause similar problems. But I checked that and it's fine.

Any suggestions/ideas? What are some possible problems as a result of high overheating?

Have you pulled the plugs and checked their condition?

Head gasket could be fubar, too.

Backfire in the manifold has been known to be caused by the following:
1.running lean...bad injector, o2 sensor, cracked manifold,
2.timing problem, coolant temp sensor, flaky TPS...
3.general ignition problems...spark plugs, wires, cap,rotor

Because you overheated, lost coolant, I would be checking for vacumn leaks, cracked intake/exhaust manifolds, replace plugs, wires, cap, rotor, clean throttle body.

Checking compression might be next...

My vote is for a cracked or warped intake valve.

alright, all good info. I'm going to check my plugs and distrubutor, rotor, etc.
How do I check if the intake valve is warped or cracked?
How do I check for compression?
Also, if the head gasket was f'd up, wouldn't I see coolant in the oil?

jeepster_259 said:

alright, all good info. I'm going to check my plugs and distrubutor, rotor, etc.
How do I check if the intake valve is warped or cracked?
How do I check for compression?
Also, if the head gasket was f'd up, wouldn't I see coolant in the oil?

Click to expand...

1: see 2
2: with a gauge
3: not necessarily

checking compression involves a compression tester, which screws in where the spark plug would go and reads pressure. so read up on that. You can also do a leak down test which shows things more accurately than a compression test and can pinpoint where a problem would come from. check into that too.

as for the head gasket F'd up, it can fail in many ways, and its not always just simple like coolant in oil.

Any combo can happen between coolant, exhaust gas and oil. from them intermixing, to external leaks, to pressurization of the cooling system from exhaust gases entering the water jackets ect ect.

My vote goes to head gasket. My cousin overheated his '01 4.0L XJ this past weekend - the coolant was a brown, muddy color and there was a good amount of solids in it. Put new fluid in it and the temperature seems to be fine, but now the truck takes a while to start up (used to start up instantly) and it has a rough idle. Pulled the fuel pump relay and cranked it over and you can definitely hear a "miss" when the truck is cranking over so at least one of the cylinders is down on compression. Try that test and see if you can hear anything amiss.

alright, thanks for the info.

What are the remedies if the head gasket is bad?

I've bought two head gasket issue jeeps this year with rough idles. The solution was letting my machinist shave the head and do a 3-angle valve job($250), or in the case of the 0331, buy a better cast head. As old_man said the valves can warp and no longer sit on the seat right(my pressure washer found the two intake valves that were culprits). Gas leaks and gets compressed(more like pressed) into the intake port if the valve isn't sealing. The head has to come off. You can get a head shipped to you for $250-275 that has been redone.

Can you give me some sources for the redone heads for $250-$275? Best I've found is $375 + core + shipping heavy/large cylinder head back to the rebuilders.

For $274. They give you the box w/ prepaid shipping to send your/an old head back.
http://cgi.ebay.com/ebaymotors/REBU...ptZMotorsQ5fCarQ5fTruckQ5fPartsQ5fAccessories
For $275 you don't have to deal with the core:
http://cgi.ebay.com/ebaymotors/AMC-...ptZMotorsQ5fCarQ5fTruckQ5fPartsQ5fAccessories
For $265 and comes w/ prepaid box for your core:
http://cgi.ebay.com/ebaymotors/JEEP...ptZMotorsQ5fCarQ5fTruckQ5fPartsQ5fAccessories

not all that likely but you could have ruined the injector o rings by getting it that hot which could cause a vacuum leak at the injectors and could cause your backfiring issue. Or you could have messed up an injector by getting it that hot and causing it to stick intermittently which would cause the rough idle and the backfire by causing fuel to pool in the intake manifold. Still, most likely candidates are a warped or burnt intake valve, but that would not be intermittent. Compression test would be the first thing as said before, but if you dont find any discrepancies there then check for a slight vaccum leak at the injector o rings.