Open Header On A Newly Rebuilt Engine

[comanche1989]

Hi Everyone, I am in the final stages of my rebuild and H.O. swap for my 1989 Jeep Comanche. To finish the swap I need to get a down pipe bent at a local muffler shop. Can I drive the newly rebuilt engine there with open headers and not worry about cracking my valves or damaging the engine? The engine will be getting broken in at this time. It is only about 5 miles there, is it safe to drive it there? Any good advice would be appreciated!

 

Thanks,

        Sam

[Jeep Driver]

I believe the concern is heat build up.........but keep moving and you should be alright.

 

Or you can spend $15+ for a piece of flex to get you there.

[Keyav8r]

We drove an Olds powered roadster with short stacks without breaking anything. Admittedly, we only drove it about 1/2 mile each trip -down the drag strip and back up the return road.

[cruiser54]

Here's my concern as I'm facing the same dilemma when I put the stroker in. 

 

The cam must be broken in for 20 minutes. I'm afraid I wouldn't be able to hear any other sounds while the straight header was blasting away. 

 

Not to mention the pissed off neighbors. 

 

I have driven broken in/used engines to the muffler shop wit absolutely no issues more than a few times in my life.

[comanche1989]

Here's my concern as I'm facing the same dilemma when I put the stroker in. 

 

The cam must be broken in for 20 minutes. I'm afraid I wouldn't be able to hear any other sounds while the straight header was blasting away. 

 

Not to mention the pissed off neighbors. 

 

I have driven broken in/used engines to the muffler shop wit absolutely no issues more than a few times in my life.

That is a big concern of mine too, since I'll have to hold it at 1500-2000 rpm for the 20 minutes. I'm gonna see if having my buddy stand by the engine bay while I'm controlling rpm will let me have a greater chance of hearing if something is up.

[cruiser54]

Nice neighbors or no neighbors? LOL. 

[Jeep Driver]

You know you can also have a flatbed deliver.........if arranged at their convenience you might get away with about $60.

[jimoshel]

I believe the biggie here is the possibility of cold air blowing back up the header and cracking a exhaust valve.

 

Possibilities;1. Tow the MJ to the muffler shop.

2.Run the engine for the required time to seat the cam Have a way of blocking off the header opening. Bucket, can with a wire to fit over the end of the header. AFTER shutting the engine off.

3. Drive it to the muffler shop 'as is' and don't worry about it. Easy for me to say. Not my truck

I would prefer No 1 because with a muffler you can hear noises from the engine better.

[nitroxsteve]

Is it a stock header? If so can't you just buy a new down pipe at the parts store?

[cruiser54]

Is it a stock header? If so can't you just buy a new down pipe at the parts store?

Probably could. Get one for a 91 or thereabouts.

[Eagle]

The other problem is that the O2 sensor won't work.

 

Several years ago on a NAXJA-NAC trail ride, one of the guys had a header tube crack. Just a crack -- but the air leak screwed up the O2 sensor readings, made it think the mix was lean, so it basically maxed out the injector cycle to the point he was pouring raw gasoline into the exhaust manifold. Turned it cherry red within a couple of minutes.

 

One of the other vehicles pulled him out on a strap.

 

DON'T DRIVE IT.

[comanche1989]

I guess I'll just order a new frontpipe then. I welded together a little makeshift exhaust to divert exhaust out the passenger side, but now that I'm hearing mixed reviews, I think I'll just be patient. Thanks for everyones help!

[Oyaji]

Put on a "stinger" pipe 3+ feet long downstream from the header and you will be fine... unless the hot exhaust stream melts/burns something or you get pulled over for being too loud. Without the pipe downstream from the header, oxygen-bearing fresh air will be drawn up the header tubes between exhaust pulses and can rapidly erode the exhaust valves if the load is sufficient to get them really hot. Pressure in the upstream portion of the exhaust system does indeed drop "negative" (that is to say, below atmospheric) between pulses and can draw fresh air all the way to the valves if the tubes are too short.

 

There is also this gem to consider: 

 

The other problem is that the O2 sensor won't work.

 

Several years ago on a NAXJA-NAC trail ride, one of the guys had a header tube crack. Just a crack -- but the air leak screwed up the O2 sensor readings, made it think the mix was lean, so it basically maxed out the injector cycle to the point he was pouring raw gasoline into the exhaust manifold. Turned it cherry red within a couple of minutes.

 

One of the other vehicles pulled him out on a strap.

 

DON'T DRIVE IT.

 

which you can balance against this:

 

 

The cracked exhaust manifold might be allowing pulses of air (and thus oxygen) back into the exhaust system, thus throwing off the oxygen sensor reading.

I concur. On start up, the ecu will ignore inputs from the O2 sensors and run off preset tables until the O2 warms up enough to provide accurate input. Then it goes into closed loop and measures fuel trims and air temperatures and whatnot. What's probably happening is the cold weather has caused the crack to increase in size and allow too much air in (too much air and the engine starts dumping fuel since it detects a lean condition) since the metal contracts. Over time, the hot exhaust gases cause the manifold to expand, sealing the crack up to a manageable level.

 

The theory of exhaust pulses should be at the bottom of your list or better yet, not even on it. I think we need to stick to the original poster's issue at hand and help him get his Jeep fixed. Practical solutions are what this community is looking for, not heady pontificating responses that just muddy the waters and inflate people's own egos.