J30 Rough engine at 2K RPM??

12-16-2003, 05:19 PM
Hi All, Need a bit of advice. I have a 93 J30, 98K miles, that is acting a bit strange. The car will start and run fine. Throttle off a stop is odd from the standpoint of it wants to jump off when you give it gas slowly. There is a hesitation and then you get a kick in the seat of your pants. I also have had a slowly developing roughness running at 2K RPM driving down the road at approx 50 MPH. Feels like a miss but the plugs are fine. I installed Bosch plugs last weekend, I am pulling them and putting NGK's back in as the performance is poorer with the Bosch. Anyhow, if I drop the car into third, and unlock the torque converter the car is smooth. Acceleration under WOT is smooth, and as long as I have the RPM's over 3K I donít have any roughness. I don't think the troopers will take it to heart when I try to explain 90 MPH is so the car will run smooth . Where do I start? I have replaced the fuel filter, and plugs. Do I need to figure out how to do a throttle body cleaning? I read how to clean the MAF, but I need to find it. I think I have 2. Advice, anyone that has experienced this?? Thanks in advance for the help

12-19-2003, 01:47 PM
Wow, I have the same problem, I have a 93 j30t, and it's doing the same exact thing. I don't know what the problem is. If you find out please post it, I would really appreciate it.


12-21-2003, 10:01 AM
Yesterday I put the NGK plugs back in, the Bosch platinums made the performance suck, and did not resolve the problem.

I pulled the Air Distribution box cause I wanted to clean the intake butterflys. I cleaned them some with carb cleaner and paper towel. Was able to get some of the nasties off. I did not spray the carb cleaner into the intakes themselves as I do not know what it will do the the bottom of the intakes or if there are hoses/fittings that it could run into.

When I was reinstalling the Air distribution box I noticed an Orphan vacuum line. Facing the car on the right side of the engine there is a line that plugs into a hole in the main air duct right after the MAF. This line goes to a electric valve that then feeds vacuum to the top of the engie. I do not know what it controls. The is a matching one on the left side duct that has a steel curved elbow. Based on the dust on the orphan fitting I bet it has been out for a while. I re connected it and took it for a drive.

The performance is better, the stumbling is still there in the 1800-2200 RPM range but is almost gone. I also have a smooth idle again and it is idling at 800 RPMs now like it should.
I may try doing some more cleaning with a throttle body cleaning kit that lets you do it on the car. Anyone have experience with the cleaning kits???

12-28-2003, 11:15 AM
Just an update. I have not done any additional work on the car, but I took it for a 100 mile drive yesterday. The engine now purrs and there is no roughness at 2K RPM. I have to assume that the vacuum line was the cause and that the plugs were fine. I had noticed that a couple of the plugs on the right side were sootier that the ones on the left when I pulled them. I did not do any cleaning before I reinstalled them. They had less than 15 K miles on them.

So tigran, I would start looking for a vacuum leak. Good luck.

05-05-2004, 07:20 AM
I have the same problem exactely problem on mine a 1993 j-30 and yesterday i find at the garage that one of my header was leaking btw the oxygen sensor was not able to read well it does me the same exactly thing so maybe its one of your oxygen sensor or one of your headers

i got my car today the problem was and i see it on the left side headers theres a little pipe that connect to the header and connect on the back of the motor the garagist tell me that to flush gasoline vapor it was leaking a bit on the left headers you can only see that when your car is on a lift

the pipe was looking ok but when the guy take out the header to check if it was break he found that little pipe approximately a 1 inch pipe that is connecting neer the oxygen sensor on the left.

check that because like you i do all the test
clean the injector
clean the maf sensor
change all plug for ngk iridium
change air filter
test all the coil

but every thing was ok for that.

now my car is running like a brand new one i feel now that 240 hp under the hood finaly

give me some news about that if you decide to repair


09-01-2004, 12:42 PM
Yes, I've had this problem myself when i first got my j30t..

since Iíve owned my car.. I only use premium fuel and add a bottle of octane boost every time I fill up (once a week), also did alot of work to it.. Like change out the air filter, change the oil, change the spark plugs...

its' been running really well ever since.

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