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1988 YJ, 258/4.2 with Howell EFI, BA10
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Discussion Starter · #1 ·
I replaced the exhaust manifold, cleaned up the intake and reattached with new gaskets and new studs/bolts, watching the dowels to make sure it’s aligned.
To complicate matters, I then installed the Howell EFI kit, applying the rtv and routing new vacuum hoses as indicated. However, when I now try to start it up it fires up beautifully and runs straight up to 4,000 rpm before I quickly shut it off.
I’ve tried capping off ports, resealed the throttle plate, checked the throttle cable, double checked the intake is seated properly, checked the brake booster.. what am I missing? I can only assume this is due to a massive vacuum leak somewhere - is there something else I should be checking?

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1988 YJ, 258/4.2 with Howell EFI, BA10
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Discussion Starter · #3 ·
Both dowels are correctly located - is there anything else to check or is it just a case of unbolting, wobbling and retightening?
 

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With the engine revving to 4K RPM at startup, I'd check the throttle linkage. Disconnect the linkage and start the Jeep. What happens if you do that?
Make sure the linkage on the throttle body is set properly. A spring may be pulling it to WOT, rather than to idle.

Good Luck, L.M.
 

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With the engine revving to 4K RPM at startup, I'd check the throttle linkage. Disconnect the linkage and start the Jeep. What happens if you do that?
Make sure the linkage on the throttle body is set properly. A spring may be pulling it to WOT, rather than to idle.

Good Luck, L.M.
I agree with this logic. A vacuum leak will make the engine run horrible, it would not make it run up to 4000 RPM.
 

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It cannot run high rpm without high air flow like an open throttle body or an unseated intake manifold

Efi arts fuel to proper mixture but air intake rate controls rpm

Which is how opening throttle body increases rpm


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It cannot run high rpm without high air flow like an open throttle body or an unseated intake manifold

Efi arts fuel to proper mixture but air intake rate controls rpm

Which is how opening throttle body increases rpm


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So, are you saying Alex's problem is his intake manifold is improperly seated or his throttle linkage is maladjusted? You certainly know more about FI than I do but I don't understand where you're saying he needs to look.
We all learn from one another, so If I can learn more about FI, I may be able to help the next guy.

Good Luck, L.M.
 

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I am saying uncommanded high rpm is from uncommanded large air flow rate into intake valves

That has to come either thru the throttle body or from a large opening to outside air past the throttle body

So if verify throttle body is closing would then have to be between throttle body and head hence the suggestion of improperly seated intake especially since he just mucked with exhaust manifold and intake and exhaust manifold share a few mounting points


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There are 2 basic forms of efi control

Mass air flow where an air flow meter is in or distal to throttle body to estimate air flow
Common types are a spring loaded door with a wiper arm on variable resistor like used in Toyota pickups in 1986 era
Heated wire in air flow path where wire resistance varies with temperature like used in 300zx of 1984 era

In these systems the internal fuel map calculates injector pulse duration based on amount of air flowing thru throttle body and rpm of engine then in close loop modified based on mixture as read by oxygen sensor(s)

Other type is what wranglers used in YJ TJ era and is called speed density
Here the pcm reads rpm and manifold pressure to estimate air flow and determine duration of injector pulse again when goes closed loop modified by oxygen sensor feedback

Unlike a carb system air thru carb does not pull in/meter fuel but instead computer has fuel map and based on sensor readings adds appropriate amount of fuel thus a carb system only has adequate air/fuel for high rpm when carb has high flow but a speed density efi setup will have adequate air and fuel for high rpm with high air flow and not requiring air to enter and pass thru throttle body air can be sucked in distal to throttle body and computer will add fuel to match


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'89 YJ 4.2 with MC-2150 Carb & HEI, 2-1/2" Ex. AX-15, NP231 SYE, Adams shafts, F&R ARB, 3
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I think what digger is saying that if OP pulls top off of air cleaner and looks down the throat of the throttle body and sees butterfly/s open linkage or adjustment screw could be issue. If butterfly/s closed, likely manifold leak either from missing plug or bad gasket seat.
I'm not familiar with the Howell EFI throttle body install, but is it possible there is a spacer missing and the plate is wedged open?

In Pic1 of post #1 it appears the Knock sensor is missing from the top of the Intake manifold and not plugged...Look just to the right of the TEE coming from the PCV line.
In Pic3 of post #1 it also appears that you are running a second PCV valve at the back of the valve cover???
 

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1988 YJ, 258/4.2 with Howell EFI, BA10
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Discussion Starter · #12 ·
Butterfly looks ok to me - just cracked open a little, and certainly not wide open. Knock sensor was removed as per Howell instructions, and the elbow from the back of the valve cover is vented to the air filter. As per Digger's comment, my assumption is that the EFI pours fuel in to match the airflow, so I'm going to just have to take everything off again and see if the intake manifold was somehow not tight up when I replaced the exhaust manifold.
 

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1988 YJ, 258/4.2 with Howell EFI, BA10
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Discussion Starter · #13 ·
Actually, did I get this wrong? Is the butterfly supposed to be completely closed at idle? This is what it looks like at the moment:
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'89 YJ 4.2 with MC-2150 Carb & HEI, 2-1/2" Ex. AX-15, NP231 SYE, Adams shafts, F&R ARB, 3
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That's more than my sniper. And pretty much any throttle body that I've cleaned kn my other vehicles.
Before you dismantle everything, humor me and put the knock sensor back in and give it a whirl.
 

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Discussion Starter · #15 ·
Curses! I knew I had to be missing something obvious but I didn’t think to check the “new” part. The new throttle linkage is totally hitting the intake manifold fitting that the brake booster connects to. Let’s address that and see if it’s fixed.

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1988 YJ, 258/4.2 with Howell EFI, BA10
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Discussion Starter · #16 ·
And there we go, a little mechanical removal and there’s clearance between the new throttle linkage and the brake booster line. Double facepalm as it actually mentions this in the Howell install instructions.

It’s idling correctly now, so time to put the rest of it back together! Thanks everyone, sometimes just needs a couple suggestions to figure out what you’re missing :)

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I replaced the exhaust manifold, cleaned up the intake and reattached with new gaskets and new studs/bolts, watching the dowels to make sure it’s aligned.
To complicate matters, I then installed the Howell EFI kit, applying the rtv and routing new vacuum hoses as indicated. However, when I now try to start it up it fires up beautifully and runs straight up to 4,000 rpm before I quickly shut it off.
I’ve tried capping off ports, resealed the throttle plate, checked the throttle cable, double checked the intake is seated properly, checked the brake booster.. what am I missing? I can only assume this is due to a massive vacuum leak somewhere - is there something else I should be checking?

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I had the same issue with engine racing after replacing exhaust manifold, turned out didn't have intake manifold aligned properly. The manifold was setting slightly on front alignment button. You can check with mirror and good light. Good luck
 

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I had the same issue with engine racing after replacing exhaust manifold, turned out didn't have intake manifold aligned properly. The manifold was setting slightly on front alignment button. You can check with mirror and good light. Good luck
Welcome to the Forum Gunboy36#,

Yep, manifold alignment is the most common cause of Alex's problem, that's why Digger84 suggested it right away.

Good Luck, L.M.
 
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