DOHC has the shudders @ 2k

[crazyd]

So I've chased down every last vacuum leak and mis-routing, and I've gooped both ends of the plug wires with electrolytic grease. Everything else about the engine is great, but something about 2000 RPM still isn't. I get the shudders under load from about 1700-2300, noticeable mostly in 5th gear where it feels like an unbalanced tire - except it stops when I let off the gas, or if I drop into 4th or 3rd to raise the revs. I'm running OEM 7mm wires and Bosch platinum plugs, everything is stock. I've heard recently (after I'd already bought & installed them) that the Bosches aren't so great for these engines, but is this the kind of result they usually produce? What else should I check?

[Identity Crisis]

My GTP that I just got does that too and the kid that I bought it from told me he installed the platinum plug.

Now I am wondering if it is the plugs that makes my car shudder at about 1700RPM.

[smokey]

So I've chased down every last vacuum leak and mis-routing, and I've gooped both ends of the plug wires with electrolytic grease. Everything else about the engine is great, but something about 2000 RPM still isn't. I get the shudders under load from about 1700-2300, noticeable mostly in 5th gear where it feels like an unbalanced tire - except it stops when I let off the gas, or if I drop into 4th or 3rd to raise the revs. I'm running OEM 7mm wires and Bosch platinum plugs, everything is stock. I've heard recently (after I'd already bought & installed them) that the Bosches aren't so great for these engines, but is this the kind of result they usually produce? What else should I check?

you can nail that problem down by using a systematic approach. it sounds like your engine is ok internally, I would do this:


fuel pressure/volume test- this will tell you the condition of your fuel pump, fuel pump relay, and fuel filter- if below specs, replace fuel pump and filter- if relay is working intermittently, replace that as well

complete sensor/ECM system scan- especially looking at coolant temp, TPS, MAP/MAF, O2, knock sensor, computer/prom

exhaust backpressure test- pull the O2 sensor, insert a pressure probe, run the car- there are specs as to how much backpressure in PSI is allowable, to make sure the converter and/or muffler are not plugged- sometimes converters break up internally, and the honeycomb ends up in pieces in the muffler- the result is like running your car with a potato pounded up the exhaust pipe

check coil-wires-plugs on an oscilloscope or similar testing device, for voltage and no misses

run a can of fuel injector cleaner through the car on a full tank of premium gas- or have a good repair garage run the engine on a injector cleaning rig

if all that checks out ok, and you still have the problem, do a cranking compression test and leakdown test- but most problems can be found during the sensor/ECM, fuel pressure/volume, and exhaust backpressure tests- very rarely is it internal engine components- I realize most of these tests are beyond the scope of a driveway home mechanic, so your next move is find a good garage that doesn't cost too much- you can't get stuck replacing sensors at $75 or more each, trying to find the problem

I especially want to emphasize the fuel pressure/volume test and exhaust backpressure test- so far we have found 4 plugged converters and as many bad fuel pumps and plugged filters, and also a few bad fuel pump relays. Then these parts were changed out, the cars really woke up and ran 10x better than previously.

[smokey]

My GTP that I just got does that too and the kid that I bought it from told me he installed the platinum plug.

Now I am wondering if it is the plugs that makes my car shudder at about 1700RPM.

it's not the plugs, I've been running those plugs a LONG time, and my car doesn't do that, and it's a DOHC 3.4

have you ever had a complete ECM/sensor scan, fuel pressure/volume, and backpressure test done in it ? that is as basic as a doctor listening to your heart when you go in for a problem- it's step one

[SappySE107]

I wouldn't listen to this guy. He just joins here to start shit and most of his information is junk.

He may be ok in this thread other than the spark plug discussion but just beware.

[crazyd]

Well Smokey's been the only one to offer some good diagnostic advice so far, so thanks Smokey. I've got a fuel pressure gauge I can use to check that, but the exhaust backpressure gauge is a new one. Is there some sort of O2 bung adapter I could use to connect a normal PSI gauge to it?

[SappySE107]

The transmission/torque converter can cause a shudder as well, and there is a fixit kit for the 4T60-E for this issue.

If you have power at WOT and its smooth, you can forget about back pressure and fuel pressure/flow. If either are bad, you won't be able to floor it and produce smooth power. Bosch plugs suck in GM cars so replace those with acdelco (cheap) and see what happens.

[Identity Crisis]

any ideas on a perf shift kit for hte 4T60E?

[SappySE107]

transgo makes one and sonnax makes a more expensive kit. The sonnax is supposed to be more heavy duty for taxis and such. This is just from reading other people discuss it that use them in their shops. I was looking at upgrading my 4T60-E since it needs a rebuild anyway.

[Identity Crisis]

I just want something with some more guts and sharper shifts. I had a kit installed in my old 82TA w/THM200 and it was pretty good shifting afterwards.

[SappySE107]

Well I think the sonnax kit is more for shudder and the transgo more for performance. That helps both questions:P

[pwmin]

none of the shift kits are going to help much on -e trannies. they are more of a fix kit. the best thing is a powrtuner, but you cant control shift time and pressure on a 96 pcm, only shift mph and rpm. i had mine shifting at 7K for the track.

[Identity Crisis]

who sells the powertuner?

[crazyd]

I have a 5-speed, so I'm pretty sure it's not a problem with the 4T60-E.

[crazyd]

Okay this problem went from annoying to serious over the weekend. I was cruising down the highway in 5th and the shuddering got bad going up a hill, then suddenly it started missing badly like I was out of fuel and I lost about 1/3 to 1/2 the engine power. Checked for vacuum leaks but couldn't find any, opened exhaust at cat flange and cat is intact, exhaust doesn't seem to be clogged though it sure feels like it. It still idles smoothly at about 850rpm but exhaust at the tailpipes sounds off and is missing at a semi-regular beat of about 1-2 Hz. Checked oil and it's good as new (500 miles since oil change). Fuel filter was replaced about 5000 miles ago but I haven't checked fuel pressure yet. No SES light, no codes.