1996 Jimmy rough idle

[dbecker63]

There is at least one vacuum line that comes up from the transmission that is 7/32, but the the line coming off the black ball on my '96 was 5/32. It leads to a "T" where one line coming off is 7/32 and one is 5/32(this was the worst one). Maybe some of the lines were replaced before, but with the wrong size - just a thought.

DB

[dbecker63]

What is the seafoam you speak of??

[wildcatsua]

I asked my neighbor (professional mechanic) to borrow his fuel pressure gage. He insisted on taking it to his shop to diagnose. He said: 1) fuel pressure good at 64psi, 2) fuel injectors good as verified with strobe, 3) used propane analyzer and determined that the intake manifold gasket is leaking.

I had the intake manifold gasket fail about 3 years ago. That time I had a large water leak on the front right of the manifold. It stranded me on an out of town trip.

I believe my neighbor but am trying to understand how this works. Does this make sense to anyone? Can anyone explain how the intake manifold gasket can be leaking but not have a water leak? Is this possibly why my radiator coolant looks and smells rancid?

thx

[BlazerLT]

Yip, it could be forcing combustion gases into the coolant.

[wildcatsua]

Why isn't it leaking water like before? Can this cause the rough idle that I've been experiencing?

thx

[BlazerLT]

Could be, or it could be caused by the crappy stock rad cap that came with our vehicles.

I would swap and replace the coolant.

The rough idle could be because of many things including needing a tuneup etc...

[blazes9395]

The leak your neighbour found was a leak at the intake (intake ports) part of the gasket - not the coolant passages. If you have a leak there, it will defiently give you a rough idle and a misfire because it is sucking in air, causing a really really weak mixture at those cylinders. Also if this intake leak isn't fixed, it will get worse and you will be running lean on those cylinder(s) that have the leak, further causing more problems to your engine(overheated intake valve, engine knock). The propane method works really good, and never lies, so if he found a leak with propane, thats your problem.

[wildcatsua]

Originally Posted by blazes9395

The leak your neighbour found was a leak at the intake (intake ports) part of the gasket - not the coolant passages. If you have a leak there, it will defiently give you a rough idle and a misfire because it is sucking in air, causing a really really weak mixture at those cylinders. Also if this intake leak isn't fixed, it will get worse and you will be running lean on those cylinder(s) that have the leak, further causing more problems to your engine(overheated intake valve, engine knock). The propane method works really good, and never lies, so if he found a leak with propane, thats your problem.

Thanks for the informed response. Seems like you were the only one who answered my question. You described the reason for the rough idle just like my friend - sucking air, lean mixture, etc.

Like I said, I trusted my neighbor just wanted to understand a little more. Now I've got to go fix the gasket. Any tips? Perhaps I did someting wrong last time - it only lasted a few years (~20k miles).

Thanks again!

[blazes9395]

Yes there is one tip...make sure you get the right intake manifold gaskets. The cpi gaskets are different from the TBI gaskets. They look almost identicle, but there is a difference, so make sure you get the right ones....or you'll be doing this again soon.

[wildcatsua]

My neighbor brought the Jimmy back after changing the intake manifold gasket. No change. He was pretty disappointed. I was too but can't really complain because he wouldn't take any money for all the work he did.

But I still have the problem so I decided to go after the fuel injector as previously recommended. When I pulled the plenum there was a heavy soot buildup at the front left side - right by the #2 & #4 poppet connections. And, unfortunately, after changing the #2 fuel injector there was no change in the result.

Any other ideas?

thx

[XtremeXP]

Is your engine CPI or TBI? I know the CPI's on the 4.3 vortec go bad and cause this.

[wildcatsua]

I think it's CPI but am not sure.

Updated idiot troubleshooting (by me) - the SES light did actually go out a day after I wrote the previous message. It then came back on the other day. New codes are P0102 - Mass Air Flow Sensor Circuit Low Frequency and P0175 - Fuel Trim System Rich Bank 2.

Am I chasing my tail?

Any thoughts would be appreciated.

thx

[blazes9395]

the P0102 can be coming from a disconnected or loose mass air flow connection - check the connection at the sensor and make sure it is plugged in tight.

the p0175 can be a result of the mass air flow code, as the mass air flow is not working properly so the computer is using other sources to estimate how much fuel to supply the engine.

Either way, your neighbour could have fixed the original problem, but now you have to contend with the mass air flow. That needs to be addressed. That will diffiently cause a rough idle and hesitation when driving.

Also, your '96 is a CSFI - Central Sequential Fuel Injection, its not a CPI or a TBI for that matter.

[wildcatsua]

Checked the connector on the MAF. It's good. The SES light has gone off and on twice over the last few days.

I'm not sure the intake manifold change did anything. I still had the P0302 Cylinder 2 Misfire Detected code after the gasket was changed. This went away when I changed the fuel injector #2. Now, a week or so later I start getting this MAF code. Just hard for me to believe that one thing after another keeps dying. And the engine still has a rough idle. Now it takes an extended crank to start the engine whether it's cold or hot. It will start immediately if you crank it right after shutting it off. Add in the soot build up near the #2 poppet that I saw when I pulled the plenum to do the fuel injector and I'm more confused.

Appreciate any suggestions.

thx