Bucking under 2000 rpm and idle

[jmars55]

I was hoping someone in here could help me out. I have a 97 Monte Carlo 3.4L DOHC. After it warms up it will buck slightly at idle and feels like a horrible miss under acceleration until 2500 rpm. It is good at first start up in the morning. It is very consistent. Also at a constant cruise under 2000 rpm it bucks. It will also do it out of any gear as long as the rpms fall below 2000. Once I get above 2500 it runs fine. I’m not getting any codes. Changed plugs (Autolite Plat.), wires (AC-Delco), Catalytic converter, fuel filter, Intake Manifold gasket, checked coil primary resistance and secondary, signal from control mod seems good. I will be changing the timing belt soon it’s a little over due. I was also wondering if it is alright to break the cam gear bolts loose while holding the cams with the cam flats tool? Had some suggestions about the EGR Valve might be sticking. If any one has any ideas it would be greatly appreciated.
Thanks

[Mach10]

Fouled/bad plug, or you've got a wire "leaking"

Turn the lights off, chances are you'll see some blue sparkly action from one of your wires.

The other thing is that it could be the knock sensor puking and retarding the timing too much.

[Lyle's GTP]

Definatily plugs, wires, ignition coils or ignition modual. Start with the plugs and work in that order.

I changed my wires and the problem went away. (Magnecore)

Lyle

[jmars55]

Hey Thanks for the suggestions. I was looking over the wires again and found that while checking the coils I must have switched the #3 & #6 wires. This took care of the bucking at idle but it still has a problem accelerating at low rpm. It seems OK if I go 1/8 to 1/4 in the throttle but anymore and it bogs down and misses like crazy to the point that it I have to either let off the gas and accelerate slowly until it gets over 2500 rpm or floor it and wait till it gets over about 2500. Sometimes when at WOT under 2500 rpm it will backfire. I will check the knock sensors like you suggested. I didn't get a chance to get under the car. Can I check these at the wire off the PCM or do I have to remove them to test them?

[Mach10]

Still sounds like a timing/spark issue.

Did you check your plugs? How do they look?

[jmars55]

I changed the plugs about 6 months ago along with the wires. I checked the front three and they looked fine. I will change them all this weekend (i'm waiting on the gasket). Are Autolite Platinum OK or should I go back to AC-Delco? I didn't see any arching at night but once again I could not get under the intake where most of the wires go.
Thanks

[Mach10]

I have no confidence in platinum/iridium plugs. They don't make power, and although they supposedly "last longer," they aren't any more immune to the normal plug failiure problems... Namely carbon buildup and glazing/detonation.

For the utter cheapiness of Delco plugs, the fact that they seem to work forever, and given that I tune my car up every year anyway.... Fnckit... Go with the Delco, and gap them right.

If it bucks severly when under load, also consider the O2 sensor as being at fault. What color are the plugs? Ideally you want a light tan color. Dark brown/black might indicate a rich condition... Which right away makes me suspect that your O2 sensor isn't reading right, or your EGR is plugged and it's over compensating.


Another thing you can try is to verify the advance. Slap a timing light on the mutha, and see what the advance is doing. It'll jitter a little at idle as the ECM goofs around with it, but it should swing fairly smoothly when you give it some gas.

Backfiring though definitely sounds like there's an extreme rich/lean condition.


Of course, the worst-case scenario is that your timing belt is right farked and you've jumped timing. That would suck, but make it do the things you're describing.

How does it idle? Does it miss at all?

One thing you can do is try to locate the dropped cylinder. Pull a plug wire one at a time while the car is running (DO NOT WEAR A WATCH WHILE DOING THIS! *unless you like the jolt). Lay the wire down on the intake and give it some gas. Give it some gas and note the shuddering. When the motor doesn't shake any more/less than normal with a specific wire out, you've found the bad one. Either the wire is leaking, or the plug's no good. Remember that when you take the wire off a cyl that IS firing right, the problem will get worse 'cos you're dropping more than one. Also, keep clear of the tailpipe... If it sometimes backfired before, it definitely will now, and likley shoot a bit of fire too. Stay safe!

If it's a fuel delivery problem, the above steps won't do a hell of a lot for ya

[series8217]

This sounds similar to the problem I'm having. It appears to be a weak injector driver in the ECM; it works "okay" at long pulse widths but hardly works at all for short pulse widths (i.e. low RPM and idle).
Also check for vacuum leaks, maybe around the MAP sensor. If the MAP sensor is off you will have all sorts of funky problems.

[jmars55]

The three plugs I pulled quickly looked good (tan and alittle darker at the tip). Would the O2 sensor give me a engine light? I have a very basic OBD II reader and it is showing "no codes". I'm going to do like you suggested and remove one plug at a time and check the difference in performance. I idles good now and at higher RPM it is smooth (over 3000).

Also I noticed that there is a vaccum piece that has a line from the Throttle body, by the IAC motor, and an electrical connection. It also has an open ended hose. Is says "Vaccum Purge Valve". This piece seems to be pulsing after the car warms up. Is this normal? I put my finger over the open ended hose and I can feel the vacuum pulsing. When it is cold the Valve does not do this and there is no activity at the hose. All other vaccum lines seem OK and have a consiant vaccum.

[Mach10]

The purge valve should be going to the vapor canister. If it's bee removed, then you have a gaping vacuum leak every time the solenoid kicks open.

That would be enough to throw your MAP off at part throttle, but at full throttle... Who knows? I don't think the MAF setups from 95+ even have a MAP. All I know is that you'd be sucking in air behind the MAF sensor, which can screw with things. But I'm not sure what the result would be. Plug it up (jam a dowel into the hose) and see if it makes a difference.

About the O2 sensor. The O2 sensor circuit is pretty simplistic, and most OEM O2s are so inprecise that they are more of a "too much-too little" meter, rather than a device that can give a reasonable reading. As such, the ECM won't generally throw a code unless it's WAY off. You can test it yourself by unplugging it, driving around for 10 minutes at moderate speed(ignore the check engine light), and let it idle. Hook a multimeter up to the O2, the other end to ground, and set it to read voltage at 2v range.

At idle the voltage should swing back and forth from 0.4 to 0.9 volts. If you give it gas, it should swing all the way to .9 and then start fluttering again.

Ah damnit... I might be totally backwards here. The end result is that the voltage should be swinging around all over the place as the ECM adjusts for idle trim. And it should poing around all over the place when you crack the throttle.

If the voltage sits steady at a particular voltage, either high, or low (or 0)the O2 sensor is boned.

[jmars55]

I finally got to change the plugs and check the wires. The plugs looked good ant the wires checked out OK. I didn't see any of the wires throwing any arcs. In 97 the MAP sensor is in the throttle body "arm" or "tube". I found out a little more about the purge valve. Apparently it is a two part thing there is a pruge valve and a purge valve switch. The purge valve has the open tube and also has a line that goes to the cansister. I was reading that this switch should hum or buss when vaccum reaches 0.7 in the canister. Mine seams to be clicking like a fuel injector. I can feel the vaccum at the open tube but only when the switch starts to click. I unplugged the valve and the clicking stopped but no change.

I did unplug the EGR valve and found that the throttle responce was much better at all rpms. Are there any problems leaving the EGR disconnected for now? This seemed to make miss less exagerated. I find now I can accelerate decently as long as I push on the throttle slowly then all the way down. If I try to floor it off the line it will still fell a bad miss until I let off the gas and let the rpm get a little higher.

I checked the O2 sensors They are 4-wire sensors. I wasn't sure sensors so I testing each wire. They seemed to be working well. Can I check the timing w/o taking off the cam covers (valve covers)?

[jmars55]

Problem solved. You guys were right on the money. I can't bellieve the problem. I searched up and down for a bad or leaking wire, but all the wires acheced out good and looked great only about 3 months old. When I changed the wires at that time I changed the plugs (autolite Plat.) In the last couple of days the problem has gotten worse and was missing all the time. So I decided to pull everything and really give it a goo look over. I even went as far as to borrow a friends noid lights to check the injector signal. At last I figured it might be a physical problem (i.e. bent valve, blow-by,...). So I was going to do a compression check. When I pulled the #3 Plug the whole side of the plug, at the base of the ceramic jacket, was burnt. I took the pug and broke it right at the base and the ceramic was burnt all the wat from the center elevtrode to the outside. The plug was apparenly fouling right through the ceramic to the seat. I had removed and checked the plugs when I first had the problem but they looked OK. I bought a set of Rapidfire plugs and it now runs great. Thanks for everthing.

[HayesPerformance]

I wish I would have noticed

Hey JMARS55,

I wish I would have noticed from the start that you mentioned that you had used those Autolite Platinum Plugs.

I had got a Notice that was given to many repair/performance shops NOT TO USE VARIOUS PLUGS ON GM ENGINES. Some that were on the list was:

All Bosch Platinum Plugs
Autolite Platinum

Are any brands where the ceramic goes all the way up the side of the electrode to the tip. eg. Bosch Platinum

The problem GM encountered was that ignition system's were too "hot" and would burn down the center electrode very progressively down into the center of the ceramic wall till it would start missfiring badly.

The same problem was reported from the same plugs with other car companys in all or certain models use a "hot" ignition system.

It made sence to me when I got it cause GM platinum or Iridium DO NOT have ceramic up the electrode like the BOSCH PLATINUM.

[HayesPerformance]

I FORGOT TO MENTION!!!!!!!!!!!

I had did a test a while ago with a 3.4 DOHC with a wide array of plugs. The BEST, HOTEST SPARK I got was with the Factory High performance Plugs that the 3.4 DOHC calls for from GM. There NOT platinum or Iridium. If you look at the plug they have a LONG LARGE electrode at the tip.

They are GM # ACR42LTSM (GAP.045")

I'll give you a break down on the part number by the way!

CODE DEFINITION - "AC" GM parts Division
- "R" Resistor (Sport Vehicle Plugs)
- "LT" Long Reach, Tappered Seat
- "S" EXTENDED TIP
- "M" SPECIAL DESIGN ELECTRODE





Some to note with TWO sets of ACCEL plugs I GOT MORE MISS FIRES THEN EVEN THE BOSCH PLATINUM GAVE!!!!!!!!!!!!!!!!!!!!

[jmars55]

Hey thanks! The Rapidfire plugs I got are working real good for now but I took down the number you gave. I have one question, you mentioned that these were .045" gap. All the service manuals and the sticker on the car say to use .060" gap plus for '96 - '97. Are these actually different than earlier models?