The infamous 3.1 leaky intake

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[Chris]

What's it take to replace the infamous leaky intake gasket(s)? I highly suspect the gasket to be my hard cold start problem especially when the car backfires through the intake and I can see smoke pouring out from under the intake plenum! Is it just he plenum gasket that usually goes? I went to the autoparts store and they told me there was a total of 3 gaskets. The upper intake gasket, the lower intake gasket and the plenum gasket. I had them order the plenum gasket because they had none in stock. I don't want to pull the intake off the car and then get the gaskets because I drive the car every day (other then when it's too cold to start - and then I won't be working in it if it's cold!)

I know someone on here has done this before...

[Dstanic]

I've changed the lower (and upper) intake gaskets on my old 95 GP, but that was a 3100 not a series I 3.1MPFI.  If I'm not mistaken the upper intake gaskets are more prone to leak on those engines, not so much the lower intake.  Are you losing coolant? (coming out the lower intake and/or going into the oil?)

[Chris]

Nope, only sympton is incorrect air/fuel ratio during cold starts, which when bad enough cause the engine to backfire through the intake.

[Prospeeder]

I would not even consider the intake gaskets for that kind of problem honestly. Especially the ammount of work you must do for the lowers.

Basicly releive fuel pressure, remove alternator, and disconect the powersteering pump and set aside, remove intake snorkle, throttle body linkage(or remove throttle body and set aside) undo the brakets holding the different cables and the Map sensor, remove the upper plenum, then the fuel lines to the rail, then remove the fuel rail, then remove the GM quick dissconnect *id recomend removing the soft line end of it and unbolting the hard line instead of messing with the quick connect if its not leaking, if it is leaking, them have fun removing it to replace it, they break* Then remove lower intake bolts, and remove lower intake, then loosen the rocker arms and remove the pushrods, keeping them all in order, and replace the gasket! Remeber sealer on the end block parts, as there is no gasket right there, most gasket kits come with the correct sealant. hope this helps!

I would really check for Crank sensor and Ignition module operation, as they control starting and running timing, and would cause a backfire. backfires should have nothing to do with intake leaks, as when it backfires it is fireing the mixture at the wrong time like when an intake valve is open or exhaust.

[Chris]

Screw that amount of work.

When it backfires, it's doesn't really go past the throttle body on the intake, it backfires and all the smoke comes out from under the plenum.

Crank sensor and ignition module work fine, or I'd have problems all the time. I only have a problem when the temperature drops below 40 and the car sits overnight. The car can sit for 20 hours during the day and start right up, but if it sits a few hours overnight it will no longer start. This only happens when the temp is 30* or below, 31* or more and it's like nothing happened.

[Prospeeder]

hmm alright change the plenum gaskets, those are easy. just take off the tb and take off the rear brakets and take the bolts out and your there!

[Chris]

I know the plenum gasket needs to be done, that's no problem - I was jsut hoping to not have to do the upper and lower

[Dstanic]

If you are not leaking coolant then don't worry about the lower. It's normal for them all to leak a little oil anyways.  Just do the plenum/TB gaskets and go from there.

[Chris]

It's been too freakin' cold for me to tear off my intake, but I replaced the TPS and the car seems to start with a little hesitation when it's cold out. I swapped the TPS from the '91 Euro to the '92 and vice versa. The strange thing is the Euro will still start in -20* with the (possibly faulty) TPS from the '92.

This Thursday I'm registering the Euro and will be driving that around for a little while so I can fix a few things on the '92 without needing to drive it every day.

[Prospeeder]

hey, try the Coolant temp sensor. It tells the ECU what temp the coolant is for starting and running. it wont start too good if it thinks the motors still 100 degrees or -200 degrees or somthing. There pretty cheap. its on the back of the cylinder head, fairly easy if the airbox is out of the way. Same with the Plenum air intake temp sensor. Could have similar effects.