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Old 06-07-2023, 04:50 PM   #1
dakota1820
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Default Swapping from a dohc to sohc

How hard is it and what is needed to swap from a dohc to a sohc is it a direct swap? Do I need to swap pcm anything car is currently an auto but I might as well just manual swap it while I have the motor out.
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Old 06-07-2023, 05:36 PM   #2
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Default Re: Swapping from a dohc to sohc

It is pretty much just the PCM that changes, if you keep within the same "generation" so that the fuel filter/FPR config doesn't change.

You can get a tested/"rebuilt" PCM programmed for whatever stock config you end up with for about $150; so that isn't a serious issue.
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Old 06-07-2023, 05:45 PM   #3
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Default Re: Swapping from a dohc to sohc

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It is pretty much just the PCM that changes, if you keep within the same "generation" so that the fuel filter/FPR config doesn't change.

You can get a tested/"rebuilt" PCM programmed for whatever stock config you end up with for about $150; so that isn't a serious issue.

Will the sohc not run off of the dohc pcm? The donor car is a 2002 the car getting the swap done is a 2000 built in 10/99. It doesn't have secondary air either.
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Old 06-07-2023, 08:48 PM   #4
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Default Re: Swapping from a dohc to sohc

Oh, I can't say that it won't run at all. As long as the ICM type and injector impedance is compatible with the PCM, it will probably fire up and run. However, neither the spark timing or fueling are likely to be optimum. How much can the PCM compensate in "closed loop"? I simply don't know.

Can you not get all you need, including the PCM, from the donor?

I was just pointing out that you could get a PCM for reasonable price, for any mix of engine/trans you end up with. No problems with codes that annoy or prevent passing emissions; no problems with cruise-control.

Sorry, I don't know the details of those two years; can't advise as to exactly what parts must be changed.
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Old 06-08-2023, 12:04 AM   #5
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Default Re: Swapping from a dohc to sohc

Going off memories of discussions 10-15 years ago...:

It should run with the DOHC PCM but it'll be expecting a richer fuel mixture and the redline is different (SOHC lower @ 5750rpm). Safer than going the other way as the SOHC PCM is likely to burn valves in the DOHC but the DOHC PCM expects the engine to be safe to 7000RPM and the SOHC blows up long before that.
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Old 06-08-2023, 12:54 AM   #6
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Default Re: Swapping from a dohc to sohc

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Originally Posted by fetchitfido View Post
Going off memories of discussions 10-15 years ago...:

It should run with the DOHC PCM but it'll be expecting a richer fuel mixture and the redline is different (SOHC lower @ 5750rpm). Safer than going the other way as the SOHC PCM is likely to burn valves in the DOHC but the DOHC PCM expects the engine to be safe to 7000RPM and the SOHC blows up long before that.

Since the intake is different plastic on the dohc and the aluminum one on the sohc is the wiring harness the same and fuel lines? Does all of that stuff swap over directly from one engine to the next or does that stuff need to be changed over to fit the sohc.
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Old 06-08-2023, 01:54 AM   #7
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Default Re: Swapping from a dohc to sohc

I believe the line is the same up to the rail so as long as you keep the fuel rail on the IM it should just plug in.

I believe the throttle cable is a different lenght with the TB in it's new orientation but I don't think it's an incompatible length (may have to tie it awa from rubbing on something). Try to compare how it connects on the TB, as long as it's compatible it'll work.
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Old 06-08-2023, 07:14 AM   #8
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Default Re: Swapping from a dohc to sohc

I would leave it DOHC, but put a manual in,
then do the 5th gear swap, from a .730 to a .605 ratio.
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Old 06-08-2023, 11:40 AM   #9
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Default Re: Swapping from a dohc to sohc

Remind me: what is ruined on the existing DOHC engine; no reasonable possibility of saving it or it's head, at least?
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Old 06-08-2023, 12:25 PM   #10
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Default Re: Swapping from a dohc to sohc

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Remind me: what is ruined on the existing DOHC engine; no reasonable possibility of saving it or it's head, at least?
So, the water pump was busted literally cracked all around the housing. I threw a water pump on it and it just dumps water into the oil. Literally a gallon in a minute or so. You can't even fill the reservoir. It empties into the crank case that fast. I'm pretty sure they left straight water in it over winter and cracked the block. I'm not 💯 sure but thats what it seems like.
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Old 06-08-2023, 01:14 PM   #11
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Default Re: Swapping from a dohc to sohc

So, the DOHC may be useable, and I would not be so quick to condemn the block. The timing cover alone may be the damaged part.
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Old 06-08-2023, 01:47 PM   #12
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Default Re: Swapping from a dohc to sohc

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So, the DOHC may be useable, and I would not be so quick to condemn the block. The timing cover alone may be the damaged part.

I don't see anything externally wrong with the timing cover. How would I know? There was oil in the coolant pouring out of the broken water pump. Once I threw a water pump on it I didn't get oil into the cooling system that I could tell which seemed odd but I had a ton of water going into the crank case.
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Old 06-08-2023, 03:19 PM   #13
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Default Re: Swapping from a dohc to sohc

Remove the water pump and inspect the cavity in the TC that the pump mounts into. Not sure about what you see? Post pictures, we love pictures to base our guesses on!

If you see nothing there, then remove the timing cover for further inspection. You might as well pull the head, too, to start getting some clues about the condition of all these parts before scrounging up a complete replacement engine.
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Old 06-08-2023, 04:49 PM   #14
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Default Re: Swapping from a dohc to sohc

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Remove the water pump and inspect the cavity in the TC that the pump mounts into. Not sure about what you see? Post pictures, we love pictures to base our guesses on!

If you see nothing there, then remove the timing cover for further inspection. You might as well pull the head, too, to start getting some clues about the condition of all these parts before scrounging up a complete replacement engine.

Well maybe I should have led with this but it does smoke at idle smells like oil not coolant. Compression was 140 ish. It was relatively even across the board. I figured knowing that it wasn't really worth trying to fix the engine and probably better just to source another. Though I may tear it down out of the car just for my own curiosity. 😁
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