I've got a 3.1 V6 conversion. After about 8 miles of driving, and everything has come up to normal operating temperature, the engine starts to buck. It gradually gets to the point that the engine feels like its going to drop out of the engine compartment! It does not do it during WOT. It happens during partial throttle openings at light to moderate loads. I've replaced the following parts:
IAC, TPS, O2 sensor, distributor and ignition module in the distributor, dist cap and rotor.
No computer codes have been set during this problem.
I've inspected the wires and spark plugs and they look good. They have about 7000 miles on them.
What next? I'm stumped. Should I look at the MAT sensor? HELP!!!!

I forgot to mention that I also recently replaced the coolant temperature sensor.

My 87 coupe is doing the same thing... I have no conversion or anything but it does the exact same thing. Runs fine at WOT, yet it jerks at partial throttle. I was thinking the mounts, I know my dogbone bushing is ripped. Thats why i think it is the mounts. But yea if anyone has any ideas throw em in here.

EGR valve?

What type coolant sensor..... the round connector or the weather packed 2 pin?

If it's the round one then the connector could be yucked which changes the resistance of the sensor which scrags performance.

Did you change from round to 2 pin? The new harness connector MUST be soldered when the sensor is changed from old style to new.

read the HOW TO: for mat and coolant sensors I posted a few days ago.

(NOTE: HOW TO:'s are works in progress that will eventually also be located in the cave.)

Check the MAP wires and vac line.

check both rubber elbows on the PCV valve line. If either is loose, even a little, clamp it. Don't overtighten the clamp. tighten just enough to ensure a seal. ((I need to do a HOW TO: for vac testing..... I type this allot.))

ignition coil?

Yes, that would be something to check.

Guys, thanks for the tips. I purchased the $7.99 coolant temp sensor from Autozone. It was the weather packed 2 pin style. There was come corrosion inside the old sensor. That's why I replaced it. I also cleaned the corrosion for the connector before reconnecting to the new sensor. Unfortuneately, that didn't do it. I had installed an Accel OEM coil last year; so I hope its still good.

I've had the EGR valve off and inspected it. Applying vaccum to the port moved the diaphram. At one point, I removed the complete EGR system and installed plates over the ports on the intake manifold and the exhaust manifold. The motor still bucked!
Would a Scan Tool be able to follow the function of the EGR in real time? What should I look for in the EGR cycle?

I had the same thing on my 3.1 conversion Twice). Both times the plug wires (firewall side) were melted. A lot of heat. I put in heavier duty wires. The problem went away. That exhaust manifold would get red hot when started, maybe thats why that side exhaust manifold always cracks.

I had the same thing happen to my 85SE v-6 4spd shortly after I replaced the engine with a long block. I finaly gave up diagnosing and took it to Pontiac. They finally found a bad ECM. With a new one installed it runs great now. I've put about 500 miles since then with no problems.

Hmmmmm..... if the engine still bucks with the EGR valve taken right off, then that obviously wasn't the problem.

Tie a rope around the transmission and tie the other end somewhere in the trunk it should stop bucking then. Also make sure your giving it high test pump gas the lower octane stuff doesnt sit well in the belly "a.k.a" engine. Good luck night rider!

Cragar:
A scantool such as the Autoxray will tell you the ECM demand for the EGR in percent, but this really has no direct relationship to the "actual" performance or position of the valve (ie.. it is the same even if the EGR solenoid harness is disconnected).

I would check the EGR solenoid very closely. Make sure that all the vacuum lines are unclogged and connected to the EGR solenoid CORRECTLY. You may also want to backflush the filter (the large cylinder). Disconnect the EGR solenoid electrical harness and test drive to "actually" determine if it is causing the problems.

Mike B.

[This message has been edited by mwbackus (edited 04-07-2000).]

mine was bucking after my 454 rebuild, (wait a sec I DONT HAVE A 454) LOL, actually my 2.8 was bucking and I changed the spark plug wires, distributor cap and rotor and the ignitoin coil and it fixed it.

where is this large cylinder (filter) your refering to located in the engine compartment.

quote Originally posted by mwbackus: Cragar: A scantool such as the Autoxray will tell you the ECM demand for the EGR in percent, but this really has no direct relationship to the "actual" performance or position of the valve (ie.. it is the same even if the EGR solenoid harness is disconnected). I would check the EGR solenoid very closely. Make sure that all the vacuum lines are unclogged and connected to the EGR solenoid CORRECTLY. You may also want to backflush the filter (the large cylinder). Disconnect the EGR solenoid electrical harness and test drive to "actually" determine if it is causing the problems. Mike B. [This message has been edited by mwbackus (edited 04-07-2000).]

Referring to the illustration of the EGR solenoid below, the cylinder (ie.. filter) is labeled (5). If the filter (or air inlet line) is clogged, the vacuum applied to the EGR valve cannot be released properly (slow release can cause hesitation and idle problems).

Constant speed surging, that only appears after normal operating temperatures are achieved, is very likely caused by a vacuum leak in the solenoid itself or leaking EGR system vacuum line.
https://www.fiero.nl/uploads/EGRSolenoid.jpg

Mike B.

I think you have leaking ignition wires. Spray them with soapy water to see if you can induce a leak while you are looking at them. If you spray them and the engine stalls then replace them with good GM one's. Do this with the engine under a load also (i.e. foot on gas and brake at the same time). Most of the time a bucking engine (especially under a load like when the TCC comes on) is caused by a failing or leaking ignition. GOOD LUKE.

Try replacing your plugs. Although they may have looked fine, a hairline crack in the ceramic can cause this problem. Don't forget to gap the new plugs.

Don't have a clue if this applies to your engine -- but the syptoms sound just like my 2.5L did a couple months back. It was a bad coil pack that caused problems once it heated up. The Pontiac dealer found it, replaced it, and it has been running great since...

Mike

It sounds like the plug wires are cross-firing. I had the same problem with one of my other vehicles.

I FOUND IT!!!!

Once the ignition coil heated up to normal operating temperature it was open circuiting. We replaced the old coil (a new Accel coil I might add, it only had 5k miles on it) with the stock GM coil and everything runs great.

I just thought you all would want to know! Thanks for your help!

Cool cragar now I will try that on mine. Fingers are crossed now.