4.2 problem

Topfueltommy69

Newbie
Original poster
Mar 23, 2022
1
Idaho
I have an 06 Trailblazer that came with the 4.2 originally. I put an 08 4.2 in it not knowing there was a reluctor ring difference. What can I do? Complete harness change? Screwed? Can someone help me with a correct answer to resolve this? Thank you.
 

Reprise

Lifetime VIP Donor
Jul 22, 2015
2,544
(on edit: Mods... if I'm totally off-base here (wrt accuracy)... just go ahead and remove my post. No sense confusing the guy. Thx.)

OP...
I'll preface this by saying I have the V8 in mine... but here's what I remember about the 4.2L model years... If I'm slightly wrong about this, someone will come along and correct me...


- '08 truck only interchanges with '08 engine (you already know why)
- '06-'07 interchange only with each other (I forget why, although I remember the cylinder head changed, but that in itself wouldn't be the reason)
- '02-'05 are basically interchangeable (might need to swap an accessory or two over, but otherwise doable).

So... how to fix, for you?

If you can't return / no '06-'07 engine available...

- You'd need a revised wiring harness, a 58x crank position sensor, and an '08 PCM, at a minimum

Or...

- You take out the '08 engine and swap reluctor rings (pretty sure they're like the V8; those can be swapped). Obviously, this means pulling the oil pan at a minimum, and likely removing the crank (the reluctor is a press-fit). A lot of work, especially if the crank gets removed.

You don't want to just throw the old '06 engine's crank in the '08 block (provided it's good). A lot of precision measurements need to be taken to ensure tolerances are good. A different bearing set might be needed (for sizing), and I'd probably put in new main / rod bearings anyway, even if you didn't have to use under- / oversized ones.

Possible to swap 'in-car' ? Maybe... *if* you don't have to remove the crank. If you do, then I'd rather take the engine out of the truck and put it on a stand. I wouldn't trust myself to R&R a crankshaft with the block in the truck. Maybe, with a second person under the truck with you, helping to lower / raise the crank into position without damaging it. That's a big 'maybe', though.

If the crossmember isn't removable, you have 4WD, etc., -- then it's gonna be easier to pull the engine (although the front driveshaft actually goes through a cutout in the oil pan on this truck, so you'll still be messing with the front diff to some extent).

If this is even possible, you'll most likely be moving the transmission back a bit to make room, as the torque converter attaches to the flexplate, which attaches to the crank. New bolts for the crank <> flexplate (with red loctite), as well as a new rear main seal.

I don't know how much clearance there is between the back end of the crank and the block (so that the reluctor can slip out from between.) Probably not enough.

TL; DR: It's 'fixable', but some pain will be involved, either way. Pick your poison.
 
Last edited:

Mooseman

Master Blaster
Moderator
Dec 4, 2011
22,548
Ottawa, ON
- '08 truck only interchanges with '08 engine (you already know why)
Actually 08-09

- '06-'07 interchange only with each other (I forget why, although I remember the cylinder head changed, but that in itself wouldn't be the reason)
Actually 02-07 will interchange where 06-07 have minor modifications. If really needed, an 02-05 motor can work in an 06-07

- '02-'05 are basically interchangeable (might need to swap an accessory or two over, but otherwise doable).
See above

- You'd need a revised wiring harness, a 58x crank position sensor, and an '08 PCM, at a minimum
That won't work. After 08, GM started integrating the new GMLAN whereas older vehicles were using the old CANBUS. The two won't talk to each other.

- You take out the '08 engine and swap reluctor rings (pretty sure they're like the V8; those can be swapped). Obviously, this means pulling the oil pan at a minimum, and likely removing the crank (the reluctor is a press-fit). A lot of work, especially if the crank gets removed.
It's not just the reluctor ring, it's the entire crank. It's cast in there. It has been done but not easy. You would have to pull the engine. The 58x sensor will screw in the same hole. Someone else here had done it. If you do go this way, you'll likely need new bearings and a good crank. Will also need to check clearances.

If you can, you're better off swapping the whole engine. If a yard sold it to you as compatible to your 06, then you have leverage as it was their fault for selling you the wrong engine. If you grabbed it out of a pick-n-pull, check what their return policy is. Ours here has a 30 day return policy, no questions asked, but get a gift card or voucher.
 
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Reprise

Lifetime VIP Donor
Jul 22, 2015
2,544
Moose -- given the corrections, I wouldn't have been offended if you'd just deleted my post, as I mentioned up top.

I had a feeling I shoulda stayed in my V8 lane... that seems to be confirmed, now :nono:

(thanks for the clarifications!) :biggrin:
 

TollKeeper

Guru
Dec 3, 2011
6,886
Brighton, CO
There is a 58x to 24x converter on the market. but I think it only works on V8 motors..


At the end of the day, that motor is going to have to come out, and a 06/07 is going to have to go in.

The only other option is to replace every single wiring harness, pigtail, module, computer, EVERYTHING, from a donor truck.
 

TollKeeper

Guru
Dec 3, 2011
6,886
Brighton, CO
After reading the thing for the converter, I dont see why it wouldnt work, as long as the plugs are the same. It would be interesting to try for sure!
 

rchalmers3

Platinum Donor
Jul 11, 2020
190
Irmo, SC
@Topfueltommy69 Have you made any decisions or actions?

I have a rebuilt, running 2006 engine that I'd be willing to sell and/or trade. But maybe you have already solved this issue or you are way out of my area.

Rick
 

TollKeeper

Guru
Dec 3, 2011
6,886
Brighton, CO
I think he was a one post wonder. Has not been back since he posted his original thread.

Not even to see the replies.
 

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