1996 3.1L conversion to 3.4L or 3.8L Engine

I currently own a 1996 Monte Carlo with a basic LS 3.1 Liter engine, and I need to do some engine work since there is water in my oil and I am assuming it is a cracked head or head gasket. Before doing the work I am interested in getting more performance out of my 3.1 Ls and would really like to do an engine swap to a 3.4L or a 3.8L. Does anyone have any information on what is involved for the conversion? I am trying to stick to a basic simple conversion with minimal modifications and finiancial input.

Is everything basically the same between a 3.1 OHV and a 3.4 OHV as far as external connections? OR am I looking at too serious modifications?

From reading some manuals I think that there is a significant difference between the 3.1L OHV and the newer 3.4 DOHC (which is different from the 3.4 OHV)? However, I haven't actually been able to make the "in-person" up close comparison.

Also, in conclusion, is a conversion like I'm talking about really worth the horsepower gain or are there other performace upgrades I can do to the existing 3.1L that would give me a significant power increase?

- Any info or links you could offer is appreciated.

Matt P.

From Virgina

My guess is if you have water in oil you only have a bad intake manifold gasket, a little easier to replace than head gaskets. You do not want to waste your time with a 3.4 DOHC. If you really are willing to make a conversion, go ahead with 3800 SC and 4T65E transmission. the 3.4 OHV would definately be the easiest, as it should essentially bolt up, I'm sure you'd have to change computer, etc.

Thanks for your response. I don't mean to second guess your synopsis, but how sure are you that water in my oil means that it is only the intake manifold gasket that has cracked? You could be right, and I hope you are. Why not a cracked head or head gasket synopsis though? The engine still seems to have adequate compression and power, which it might not if I had a cracked head or head gasket right? There is not really any noticable difference in performance other than there simply being foamy water in the oil, and the car goes through water quickly with no external drips. So maybe it is just a cracked intake gasket?

Second and main part of the post topic, (engine conversion) do you know what year(s) for Monte Carlo the 3.4L OHV engine was installed? The conversion to the 3.8L sounds a little too complicated since I would be changing the whole setup (i.e. transmission, engine, external wireing, hoses, computer) Also, how much of a horse power increase is there between the 3.1L OHV and the 3.4L OHV engine? And how sure are you that I would have to change the computer, perhaps just "reprogram" the one I have? Also, I'm trying to narrow things down, what else specifically do you foresee me having to replace other than the computer if I do the 3.4L OHV engine conversion? Do you think the Intake F.I. body would bolt up? One reason I am asking, is that I was thinking about buying a new engine from Advanced auto (that comes with a 3 year 150,000 mile warranty) for around $1,700.00 since my 96 Monte Carlo already has 147,000. But I don't believe their new engines come with all the fuel injection and ignition stuff that would bolt up to the newly installed 3.4L in the old 3.1L setup? I know this may be a little much to consider all at once... and I'm not expecting that you know all the answers... but perhaps I can try to narrow things down the best I can. And any information you can provide from your extensive knowledge and enthusiasm for Monte Carlos would be greatly appreciated.

Sincerely,
Matt

from Virginia

The reason I suspect the intake gasket is because it will happen to everyone who has a 3100. Almost every user on here can attest to that, especially if you have checked compression like you say and it's normal. As far as the engine swap is concerned, I'm not sure if the top end of the engine will bolt up to the 3.4 OHV. I've never tried it. I wish I could tell you more, but I'm not positive. Honestly, I'm thinking you can bolt up the rest. I personally think if you were serious about this to get a low mileage, used complete 3.4 OHV from a salvage yard. It'd be cheaper, and easier to install. Also, I'm pretty sure you'd get the correct electronics with it as well. (depending on salvage yard) I would get a reliable mechanics opinion near you about the intake gasket, but I'll be that's your problem, in which case, I'd repair it and keep driving it. (that's just me though). Whatever the case may be good luck.

I think there is only about a 20 or 25 hp gain to switch to a 3.4 OHV, also

Thanks for your response and insight. I don't know if 20 to 25 hp gain for the 3.4 OHV is worth the hassle. Do you know if there is any other performance upgrades I could make to the 3.1 or 3.4 OHV that would increase the hp even more significantly (i.e. cam, ignition, headers)? Or where I might be able to purchase engine performance parts?
Depending on what performance engine parts I can find, I might just try replacing the intake gastet first and just keep on driving. How big of a job is it to install the proper intake gasket (i.e. special tools, tricks, extra parts, man hours, etc.)?

First, www.rsmracing.com is a website that sells a lot of 3.1 parts (as well as many other GM v6 parts), and it says the approximate gain near the parts. The intake gasket isn't a terrible job, it doesn't require any necessarily special tools, and it will take a half a day probably unless you do that kind of thing often, it could take you less. You should be able to buy a kit when you buy the gasket that includes everything you need for the job, such as new o-rings for the injectors, TB gasket, etc. Like I said, I'd get somebody to physically verify that's the problem before digging into it, but again, it's not a terrible job if so.

Thanks for all you help. I, timidly, took the engine apart this past weekend. However, I felt confident in doing so that it was probably going to be the intake gasket. As I tore the engine apart I got to the intake manifold gasket and found that it was cracked on the drivers side and passager side coolant ports. The inside of the engine was a mess. There was a thick oil/water sludge caked throughout the engine. I spent most of my time hand-wiping the caked parts that looked like a thick solution of "chocolate milk." I hesitatingly put the engine back together without replacing the head gaskets or checking for cracked heads. Oh, I also got "o-rings" for the fuel injectors, valve cover gaskets, engine flush, compression treatment solution, and high-temp engine paint. I painted the intake (upper and lower) and the valve covers to match the color of the car (just to add my personal touch - not necessarily needed to fix the problem though). I put everything back together, and after fixing a few sliced "o-rings" which caused a fuel leak. I finally got it finished after 2 days. I'm not that experienced of a mechanic, I followed my Chiltons manual to the "T" and I spent meticuluous time paying attention to detail and cleaning the parts that were dirty.
The car runs good, and I put it to the test by driving it 6 hours back home to Virginia. There is no water mixing in my oil now, and I haven't heard any "Knocking" yet from the lower bearings. I like my car, but after reading through the process of replacing the lower bearings for this car I think that repair would be beyond my capabilities. As for now, I'm just going to continue driving my car to see how many more miles I can get out of the 3.1L. When that goes I think I'll try to upgrade to the 3.4L OHV, for the practically "bolt-in" conversion and the common performace parts that one can buy for that engine.
Has anyone ever replaced the lower bearings for a model like this (1996 Chevy Monte Carlo LS 3.1 L engine), and how extensive is it?
Thanks for all your help, and insightful information.
-Sincerely,
Matt P.
Virginia, USA

I don't think th 3.4 L is any better for reliability, especially when it comes to intake gaskets. The 3.4 's that are in the GM minivans,( Chevy Venture,Pontiac Montana, Olds Silhoutte) were nortorious for leaking intake gaskets. All one needs to do is look through these forums to get the feel about the problem. I also own a vehicle with a 3.8 (Buick Lesabre) and am very pleased with that engine. In talking with other owners of vehicles with 3.8 engines, they also are pleased with the reliability and ease of maintenance of these engines. In the future, if I was looking at 2 different vehicles to buy and one had a 3.4 L engine in it , I would probably shy away from purchasing it based on not only my experiences with it, but also the experiences of other people that I know. In fact Chevrolet is replacing the Venture this year with the Uplander and will use a 3.5L engine instead of the 3.4 L.

Glad to hear you put it back together and all is well. '97Ventureowner does have a good point. Pretty much all GM 60 degree v6's have an intake gasket problem, so if you go with a 3.4 eventually, you'll go through the same thing. 3.8's have less problems in that respect, but I don't thing your tranny will bolt up to it, so you'd have to replace that as well. As far as bearings go, If the engine isn't in the car, it's easy. I've never tried it with the engine in the car, but I believe you can't even remove the oil pan with it in the car. If you did a good job flushing the engine I doubt you'll have to worry about it. It's a good idea to change your oil very soon (like after only 500-1000 miles) after changing your intake gasket, since you had water in your oil previous - just to make sure it's totally good and clean.