Going to replace the intake manifold gasket on a ‘96 with the 3100 as the car idles anywhere from 1k to 2k rpm with no consistency to the problem and I’ve already changed the IAC, TPS, MAP, and MAF to no avail. Which gasket do I get? I see some are 1st design without guides and some are 2nd design with the guides, but even then I’m not sure which is a metal gasket and which is rubber. I know I need new manifold bolts, anything else I should check on? Reading the live data shows that the temp sensors are good so I don’t believe those are throwing the idle off. Should I do the head gaskets while I’m there incase they’re on their way out as well? Seems like these cars had a lot of gasket trouble.

 

Not trying to be rude, but has this car ever run right?

I see you have a thread from Jan 2021 with idle issues and another in May 2022 with the same thing. Just trying to understand if this is an ongoing issue or something brand new that came up again after fixing it previously.


I can't be of help there as its been too long for me. My 96 came with a 3100, but I did the LIM gaskets way back in probably 01 or 02. I just used whatever autozone gave me.

If you're worried about it, I'd buy a master gasket kit (so you can replace every single gasket you touch) - Mahle sells one on rockauto under the intake manifold gasket section for about $75.

It comes with one type of LIM gasket, but the other one (also Mahle brand) is only $7 by itself. Id just get both and use what matches. Youll only be out the cost of a happy meal.

Just curious, did you do any kind of checking to see if the intake gaskets are the culprit? Smoke test / propane test, etc.

How much mileage? Have you done any kind of testing to check them?

Its a good bit more labor (and more risk of screwing something up) but you're never going to be this close to them again (hopefully).

 

That fluctuation might be a vacuum leak. Bumpin' gave some suggestions, one I have used is with the car running, mist starting fluid anywhere that has a vacuum line or seal, even try to get it under the upper intake around the fuel injectors, listen for a specific change in idle (as it's sucking in the starting fluid). If you find a spot, that becomes the source to focus on. Also, are you able to get any scan data off the PCM? Such as misfire reports or any check engine codes? I have seen some of this stuff being worn plugs, poorly gapped plugs, failing ignition coil. All take a little work to diagnose.

I have also seen the 3100 handle a blown head gasket a bit differently, forcing compression gases into the cooling system and resulting in over pressurizing the cooling system. When you park the car over night and the pressure bleeds off, a little coolant gets in a couple cylinders and the first minute or two idle is weird and it blows out (but not really leaving any evidence like the common white smoke).

If you are going to do intake gaskets, you might want to consider due to age and level of work going down to the head gaskets. You are halfway there.

As for intake gasket kits, I recommend FelPro metal core gaskets. I always have to look at the product pictures (there are going to look gray/silver usually with black molded rubber seals). I have never replaced upper/lower intake bolts when changing gaskets. Heck, I have re-used head bolts on every head gasket job I have done (it's not a precision race car). You certainly can replace them (I would worry about head bolts more than intake bolts).

 

I bought this car from a cousin up the road a few years back and it ran fine on the way home and threw a fit there after. It has sat since I’ve done little to the car due to lack of free time but decided either it will run right or be scrapped/parted out.

I’ve doused every corner I could get to with starting fluid with no change. I’m thinking of trying the gasket as it seems to be the issue for everyone else, and just as a side project for the experience. I haven’t done a compression test yet but I will give it a shot just to see, though it doesn’t act like it has lost any compression, I believe it’s somewhere between 230k to 250k but knowing the old guy who bought it new he took good care of it.

 

That kind of mileage makes the head gaskets a tough decision then, especially since you don't seem set on keeping the car anyways. Its certainly in the tail end of its life, so you hate to put too much time and effort into it.

Given the situation, I'd probably just do the bare minimum gaskets to get it functional again. Id still buy the entire top end gasket set though so you have a replacement gaskeg for everyone you take off - but I wouldnt go digging for more work to do.

 

No codes, though I did get one for a misfire on cylinder 6 but it was only that one time and since has stopped skipping. Cars are odd. I’ll likely do plugs while I’m at it. I can’t seem to find any gaskets that specify that the material is metal, only molded rubber, but considering the age I’ll go with whatever I can get since it’ll likely outlast the car anyway. On a side note, if I flush the coolant system very well can I use regular antifreeze instead of the dexcool?

 

As I said, I look at the pics. If the lower gaskets are all black, they are plastic. And sure, it will probably outlive the car the way you listed stuff about the car, but you never know. I prefer spending a couple more dollars to get the better ones. I pulled up your 96 Monte on RockAuto. Here are FelPro metal core gaskets (as seen in the pics, metal color with black rubber seals):
https://www.rockauto.com/en/moreinfo...t=5424&jsn=994

Now as for dex vs ol' green, yes you can flush out all the dex and run green if that is what you want to do. If you concern is the myth "dex destroys gaskets", let me give you some info.
1. - GM LOST a class action law suit on this in the 90's. Yes, early dex was found to be damaging.
2. - Dex that is bought post 2000 is reformulated
3 - Almost all green coolant will say "compatible with everything". I personally have put green into a dex car. Biggest issue, the color (looks brown). Performance, just fine. My current 3400 powered daily driver is a victim of that (kind of an experiment, drained the coolant, filled with green, curious if the next time I drain and fill with green, will is stay green).
4. - I have worked on a good share of 3100/3400/3800 engines and lower gasket failures. I have seen early 90's 3100's plenty of times that ran nothing but green, even from the factory and lower gaskets fail from 80k-120k miles. Real reason, plastic gaskets are just crappy.
5. - I have a friend with a 1994 Formula Firebird, it has used Dex since the day it was assembled and he bought it brand new (this means it is that "corrosive" formula. He still owns it, car has less than 100k and sure he has had to do services and routine coolant flushes and continued using dex (I believe usually "Prestone with Dex"), zero issues with the car, no coolant problems.
6. - In 2021, I bought a 2000 Monte (the one I call "the black monte") that has over 200k miles. This spring I changed the lower gaskets not due to a coolant issue, it was actually oil bleeding out. The original owner (who had it since it left the factory) claims those gaskets have never been changed. I was stunned. They were in WAY better shape than my blue monte with about 60k miles! The original owner informed he heard AC Delco dex is a bit different from "Prestone with Dex". As a result, this year I had to drain coolant on both the blue and black Montes. I used this coolant and mixed with distilled water (I am usually the guy using straight tap water, but given what I just saw, I went this route). Here is the coolant I used:

And here is a link where I show pics of those 200k intake gaskets:
https://montecarloforum.com/forum/sh...e3/#post728744

Now, ask 100 people and you will get many anti-dex people out there. Personally, I would think if GM lost a class action lawsuit once, they would want to avoid a second round of that, hence reformulating. If it doesn't show, I personally feel it doesn't matter which you use. The one thing I do NOT subscribe to is the idea Dex is good for 100k miles or 10 years. Has it survived that, sure, should it, eh, I will say probably not. I would recommend routine coolant flushes no matter which coolant you use.

 

Very informative, thanks. As for the gasket it just threw me off that it says fits either or, with or without the guides. Guess it comes with both gaskets or a combination design. Either way I appreciate the help.

 

Back again guys. Finally got around to finishing the LIM gasket job, it was definitely on its way out the door. However it still idles high, sometimes starting out okay for a second or so then climbing it’s way up. No clue what the matter is. Sprayed many places with to look for vacuum leaks, no change in the idle. I’m at a loss for what the issue could be.

 

Check the throttle position sensor?