running very lean!!! Is it from a cracked Exhaust Manifold

running very lean!!! Is it from a cracked Exhaust Manifold

GSMC

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San Diego, CA
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1981 CJ7 258 (4.2), 4 speed
Hello everyone, I just joined so let me know if I did not do something required. I am new to the jeep world, bought my first jeep as a project. So far, there has been a lot of problems, but I need help with this one. I can not get the AMC 258 i6 / 4.2l to idle good or run right. It is very lean, and when I spray carb cleaner near the intake manifold where it bolts up, it revs up. I assumed a bad gasket, but after I pulled the intake and exhaust, I found a crack in the exhaust manifold in the center where all 6 collect. I did not find any abvious gasket leaks or problems with the intake. My question is, can a large crack in the exhaust manifold cause a vacuum leak to be sucked back into the exhaust valves? I have heard of this being possible. Has anyone else seen this, the exhaust manifold was white on the inside due to lean run. The low speed screws are out 3.5 turns each. Any thoughts?
 
No expert but can't see that being the case unless it were the intake manifold but that's obvious. The reving up when sprayed may have been a poor seal with the intake/head as this a problem area. Sounds like you need to replace it anyway with a good one, reinstall with a fresh gasket and go from there. just my 2c.

Oh ya...Welcome to site!
 
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Agreed, I am looking for a used or new exhaust manifold now. Any suggestions on headers or stock manifold?
 
Hello everyone, I just joined so let me know if I did not do something required. I am new to the jeep world, bought my first jeep as a project. So far, there has been a lot of problems, but I need help with this one. I can not get the AMC 258 i6 / 4.2l to idle good or run right. It is very lean, and when I spray carb cleaner near the intake manifold where it bolts up, it revs up. I assumed a bad gasket, but after I pulled the intake and exhaust, I found a crack in the exhaust manifold in the center where all 6 collect. I did not find any abvious gasket leaks or problems with the intake. My question is, can a large crack in the exhaust manifold cause a vacuum leak to be sucked back into the exhaust valves? I have heard of this being possible. Has anyone else seen this, the exhaust manifold was white on the inside due to lean run. The low speed screws are out 3.5 turns each. Any thoughts?

In theory a exhaust leak can make you run lean.
Kind of like adding a header you have to richen the mixture.
That however does not account for the rev. increase with carb spray.
Although if you sprayed enough it was sucked in at the card to intake or had a leak you missed.
 
Agreed, I am looking for a used or new exhaust manifold now. Any suggestions on headers or stock manifold?

I'm running Clifford Research 6 into 2 ceramic coated headers into a reversed
Terminator muffler 2- 21/2's from the headers into the muffler & 3" out.
It works great & it's a lot easier than trying to run H-piped dual exhaust, but the same scavenging.;)
 
I sprayed while all around the intake and it revs up everywhere I spray from 1-6 intake ports, all bolts were tightened and still the same. Could there be a hidden crack in the intake? is that uncommon? Also, where do I buy jets for the stock 2brl carb?
 
I sprayed while all around the intake and it revs up everywhere I spray from 1-6 intake ports, all bolts were tightened and still the same. Could there be a hidden crack in the intake? is that uncommon? Also, where do I buy jets for the stock 2brl carb?

Sorry, I'm not the best on stock parts.
I'm an old Hot Rodder, All my carbs are Holley's.;
My wifes rig is the AMC 258 i6 / 4.2l , it Has a Offenhauser Dual Port Dual Plane aluminum 4 brl. intake and a 390 CFM Holley vacuum secondary carb.
 
I sprayed while all around the intake and it revs up everywhere I spray from 1-6 intake ports, all bolts were tightened and still the same. Could there be a hidden crack in the intake? is that uncommon? Also, where do I buy jets for the stock 2brl carb?

I've seen plenty of cracked exhaust manifolds.
Not intakes though, your exhaust was probable cracked due to the high heat from driving on the highway lean for long time. I would wager your exhaust valves could use some love too.;)
 
your exhaust was probable cracked due to the high heat from driving on the highway lean for long time. I would wager your exhaust valves could use some love too.;)

I would take the head off and see what the pistons looked like also.
Running lean can do a lot of damage.
 
I'm not sure I can get my head (no pun) around an exhaust leak causing a lean-burn condition, even if it's a pretty severe exhaust leak, even if it's before an O2 sensor. No, wait - if it's before an O2 sensor it's possible a tiny bit of outside air could get sucked into the exhaust & give the O2 sensor a false high-oxygen reading.

I suspect a warped intake manifold, though. If it's not severe, you could flatten it by drawfiling or have it milled/ground flat. Check it with a straightedge, every direction you can - it should be nearly perfectly flat on the gasket surface.

Were it not for the spray changing your idle speed, I'd suggest a carburetor kit. The reaction to your spray, though, sure barks "vacuum leak at the intake manifold".
 
Thanks for all the info, I have rebuilt the carb, and the engine is strong and was rebuilt recently. I just assumed as a gas turbine tech, I would be able to examine the gasket surface to determine if it was compressed and sealing, next I will try the straight edge to determine in I have a warped intake. Great advice.
 
One question I have, I am knowledgeable on O2 sensors affecting late model FI systems due to there input to the computer, but my assumption is that the 81 CJ just uses it for a fault light or check carb light. Is this correct? Not quite sure what the stepper cam does, maybe it is part of this, any help?
 
Thanks for all the info, I have rebuilt the carb, and the engine is strong and was rebuilt recently. I just assumed as a gas turbine tech, I would be able to examine the gasket surface to determine if it was compressed and sealing, next I will try the straight edge to determine in I have a warped intake. Great advice.

I don't see how a cracked exaust manifold could make it rev up when you sprayed around the intake. So I think even though you can't see where it's leaking, it must be leaking.
 
I would like to summerize what everyone is saying by.......If you have a vacuum leak at the intake gasket (which you did) your engine will run lean because it cannot pull the needed gas thru the carb. Your engine reved up when you sprayed carb cleaner on it because the motor sucked it in the combustion chamber and burned it.. Your engine running lean heated up your exhaust and cracked the manifold. Replace the manifold and get a good seal on the intake and it should run much better. Yes the O2 sensor is is for a feed back basically useless:chug: Let us know how it goes
 
your lack of seal at the intake is causing you to run lean get a good seal and a new exhaust manifold it should run much better....you cant suck fuel outta the carb with a leak.... you running lean cracked your exhaust.....the O2 sensor is for feed back/ useless:chug: let us know how it goes. P.S. it revs up because you sucked the carb cleaner into the combustion chamber and burned it
 
Got it, I am living on a Ship in the Navy, when I get off, I will check the intake for warpage and I have a used manifold for $25 I will buy, new gasket and should be good to go. Should I use any additional sealant to the gasket? I am concerned that if the intake is not true and the exhaust is, will there be issues getting it to pres against the head due to the disign of the bolting method. I mean if tightening the set, the exhaust hits the gasket first, and the intake barely gets any tightening force. Is this possible and is this where sealant would help? Sorry, but I like to ask alot of questions, it is my nature.
Don
 
I'm not sure I can get my head (no pun) around an exhaust leak causing a lean-burn condition, even if it's a pretty severe exhaust leak, even if it's before an O2 sensor. No, wait - if it's before an O2 sensor it's possible a tiny bit of outside air could get sucked into the exhaust & give the O2 sensor a false high-oxygen reading.

I suspect a warped intake manifold, though. If it's not severe, you could flatten it by drawfiling or have it milled/ground flat. Check it with a straightedge, every direction you can - it should be nearly perfectly flat on the gasket surface.

Were it not for the spray changing your idle speed, I'd suggest a carburetor kit. The reaction to your spray, though, sure barks "vacuum leak at the intake manifold".


I said in theory.
I know when I put new free flowing pipe on my dirt bike.
It would backfire when I backed off the gas.
Untill I re-jetted the carb richer.;)
I did say I thought the vacuum leak caused the crack in the first place.;)
 
Got it, I am living on a Ship in the Navy, when I get off, I will check the intake for warpage and I have a used manifold for $25 I will buy, new gasket and should be good to go. Should I use any additional sealant to the gasket? I am concerned that if the intake is not true and the exhaust is, will there be issues getting it to pres against the head due to the disign of the bolting method. I mean if tightening the set, the exhaust hits the gasket first, and the intake barely gets any tightening force. Is this possible and is this where sealant would help? Sorry, but I like to ask alot of questions, it is my nature.
Don


From one surface squid to another, welcome to the site Chief :chug:

Let's see how many of your questions I can ask for you all at once. First, CJ's don't have a check engine light, of any kind, never did. When you go buy a new gasket set, make sure you get a good FEL-PRO set, spend the extra money, it'll be worth it. NAPA part #FPG MS90949 $21.49. You're going to get two gaskets in the set, a larger one that covers all 12 ports (6 intake & 6 exhaust) and a smaller oddly shaped gasket that only goes around the intake ports.

I highly suggest to NOT use any kind of sealant, if your manifolds aren't warped and you get a good set of gaskets they will seal. I'm going to believe that your exhaust manifold may be warped, the AMC intakes are notorious for warping and cracking, the intake manifold, not so much. Also, the exhaust manifolds can be bought new, the only new intake manifolds are high performance 4 barrel set-ups. When you get the intake and exhaust manifolds and new gasket set all lined up and ready to to tighten, don't forget to follow the torque sequence and whatever you do, don't overtighten them!!! Only torque them to (I'm almost positive) 25 ft/lbs. Let us know how it goes.
 
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sorry Elwood but my 1981 CJ7 does have a "CARB CHECK" Light that comes on when I first turn the Ignition on. It should go out after the Engine is Running.
Check the Vacuum Lines, my 1981 CJ7 has miles of them and a cracked hose could create this problem.
 
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sorry Elwood but my 1981 CJ7 does have a "CARB CHECK" Light that comes on when I first turn the Ignition on. It should go out after the Engine is Running.


Where is this light? The only two warning lights in the speedo cluster are "BRAKE" and "4WD". I'm not doubting you, just never heard of a "CARB CHECK" light on a CJ gauge cluster before. Does this have to do with your EFI system?
 

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