3.1 question (Page 1/1)
SR-71 JUL 20, 08:24 PM
Pardon me, but I have scoured the interwebs, and I cannot find a definitive answer. Just people making unsubstantiated claims.

Can anyone tell me, is the '93/'94 3.1 an interference or non interference engine? Can you back up your answer? Like I said, there seems to be no proof one way or another that I can find, and I want to know if I should make the effort to replace a broken timing chain or get a replacement boat anchor.

Thank you in advance!
pmbrunelle JUL 20, 09:25 PM
When I took apart two Fiero 2.8 engines, and a Beretta 3.1, I found the timing chains quite stretched, even with relatively low miles (supposedly) since previous rebuilds.

I would change the chain if you have the chance.

While the timing chain is out, bring two pistons to TDC, place wax on the piston crown, and rotate the camshaft through a cycle. Take photos, for the next person who comes along with the same question.
SR-71 JUL 20, 10:12 PM

Originally posted by pmbrunelle:

Thanks for your reply. I'm assuming the chain is already broken, since I can crank the engine but the rockers don't move. On previous engines I've rebuilt, they were OHC interference engines and the valves got destroyed when the timing belt broke. This is my first broken timing chain (actually it's my step sons car). What I need to know, is will these valves be destroyed too? I don't want to add a bunch more work taking the heads off if I don't need to.

If yes, I can tell him he needs to get another engine before I start, so it doesn't sit in my garage taking up room with the motor out...
Raydar JUL 20, 10:30 PM
You're absolutely correct. Most everything that I have found seems to be stolen from another site. Not surprisingly, most all of the sites are the same.
The only engines they seem to care about are engines with timing belts. They don't consider engines with timing chains.
Some of the data also seems to be incorrect, unless I have misunderstood. The 3.4 DOHC is non-interference through 95. 96 and 97 are supposedly interference. They are ALL mentioned.
Sorry. An obvious tangent.

Not really the answer that you are looking for, but I have never known anyone - at all - that claims to have had to replace any valvetrain pieces, due to a failed timing chain - ever - on a 60 degree V6. To include the 3.1. And I've heard of a few broken chains. (Of course, someone may jump in and tell me I'm wrong. And if they do, that's great. We'll both learn something.)

With that said... I had a 350 that broke a chain or jumped a gear. Valves got really friendly with pistons. I'm pretty sure that the only casualties were pushrods. Some were obviously visibly bent.
The point being... maybe the valves in a 3.1 would survive, at the expense of the pushrods. A compression / leakdown test might provide the best proof.

Probably a lot of useless info, but it's something.
pmbrunelle JUL 21, 08:34 AM

Originally posted by SR-71:
I don't want to add a bunch more work taking the heads off if I don't need to.

The camshaft is in the engine block, and the timing chain just does a short loop between the crankshaft and camshaft. The front engine cover is removed to access the chain, but the heads can remain in place during this operation.

I don't think it's big enough of a job to warrant getting a second engine to work on.

The most difficult steps are to remove the harmonic balancer and crankshaft sprocket, and to reinstall these items. Having a plan/tools to tackle these steps in advance will help prevent delays.

[This message has been edited by pmbrunelle (edited 07-21-2021).]

fierofool JUL 21, 09:55 AM
GM produces engines with similar nomenclatures and they are frequently confused. 3100 and 3.1, 3400 and 3.4, 3800 and 3.8. Some had iron heads and some had aluminum heads. Engine blocks with aluminum heads can be converted to iron head engines in many cases, but the pistons must be changed in some applications.

Maybe this will help you. http://www.gafiero.org/bbs/index.php?topic=3955.0
SR-71 JUL 21, 11:09 AM
Thanks everyone for the replies. I hadn't even thought of a leakdown test. I can remove the rockers and that would tell me quickly, plus I could check the push rods. Again, thanks!

I'll be sure to post what I find. I suspect that the valve train will be fine, but maybe some one in the future will have the same question.