Rough running

[rush91]

"Threw the scanner on it this morning. The car was bucking 2 minutes after leaving my driveway. If at any time I open the throttle close to full throttle it smoothes out"


What do you mean by bucking? I only ask because I hear that term a lot on here. What I'm picturing is the car is jerking or bucking, kinda like the fuel is being cut off? Does this happen when you let off the gas or are into the gas? Any gear? I saw you mentioned it happened mostly under 3000RPM........

[Paul Workman]

Originally Posted by 2ljd

Paul,

If you don't mind me asking, what scanner/program did you get?

Mine is a discontinued AutoXray unit - for which I also was able to purchase the computer program which produces graphs and charts from the data registered in the unit.

I believe AutoXray is "no more". I also purchased a Snap-On MT 2500 "in case" the AX dies. However, I need to also buy the appropriate cartridges to read OBD-1 duties.

[tomcat]

Hi, have you checked your coil packs or vaccum hoses?
Manfred E....
91 blk/blk #906

[Billy Mild]

Originally Posted by Paul Workman

Intermittent issues are the worst. I had a similar issue with my 95 LT1 car; took it to the shop, only to have them say: "No problem found". But, before I reached home, the rough running cropped up again.

That issue is what prompted me to buy a scanner and do datalogging on the fly. That's when I found one of the O2s was shorting out, intermittently, causing the ECM to flood that side of the motor with fuel. BLM on that side went nuts too - along with injector dwell time on that one side.

Point is, if you can datalog in real time, you might be able to catch what is going on. Mind you, often it didn't throw a code either.

But, have patience. Once you have your "Ah-HA!" moment, all this trouble will fade away to be forgotten. Trust me. (I've been there, done that several times!!).



Paul.

I have been driving with my MT2500 on it most of the time. The only thing I can find is BLM is way off between the left and right side, and my O2 sensor seems to sometimes hang for a second, then go right back to normal(which is all over the place), my IAC counts are occasionally too high.

Originally Posted by rush91

"Threw the scanner on it this morning. The car was bucking 2 minutes after leaving my driveway. If at any time I open the throttle close to full throttle it smoothes out"


What do you mean by bucking? I only ask because I hear that term a lot on here. What I'm picturing is the car is jerking or bucking, kinda like the fuel is being cut off? Does this happen when you let off the gas or are into the gas? Any gear? I saw you mentioned it happened mostly under 3000RPM........

Bucking or cutting out. This morning it was so bad the car kind of shut off then came back to life. Its like the car lost power that time. Most of the time it feels as if the banks of the motor is fighting itself, but if I go WOT it clears up. This happens with the KEY ON or OFF.

Originally Posted by tomcat

Hi, have you checked your coil packs or vaccum hoses?
Manfred E....
91 blk/blk #906

Is there a way to check the coil packs WITHOUT pulling the plenum? I have been trying to find something in the FSM.

I will give a brief history on the car. When I purchased it the car did not run well. It had a few stumbles and didn't run clean. Another member and I went through and replaced all the fuel injectors, spark plugs, and wires. Along with that I cleaned all the grounds. Fixed a lot of small vacuum leaks. Replaced fuel filter. This was 2 years ago.

Last year I replaced all the coil packs, and had to re-fix a few vacuum leaks. I have ran a few tanks of seafoam as well in the gas. No change.

This summer I have put on Longtube headers with Z06 cats, xpipe in the exhaust, new O2 sensors(Standard Motor brand), and a tune from Marc Haibeck. After I had these things done the car ran pretty dang well. It was smooth pulled great in any condition then out of no where I noticed it was running bad almost like it was sputtering when on the highway. That was last week and here I am with the same issue.

[Billy Mild]

I did some fuel pressure testing today. I bought an Actron Fuel pressure gauge. My results were FP1 static pressure with key on for a few seconds 44 PSI, FP2 with key on for a few seconds was 48 PSI on the dot. FP1 spiked to around 50 PSI then dropped down to 44 PSI. FP2 went to almost 52 PSI then dropped to 48 PSI. Would a clogged fuel filter cause a pressure difference?

I did the idle test with just FP1 and the pressure was right at 40-41 PSI.

It seems to bleed off after 10 or so seconds. The FSM says this is normal. I would still like to do the test for FPR test by jumping the 12v fuel pump test leads. I don't know where that jumper lead is located.

TL;DR- FP1 44 PSI, FP2 48 PSI for static fuel pressure test.

Is 44 PSI okay or should I go all anal and replace the fuel pump due to it not meeting the spec in the FSM?

[dredgeguy]

I had similar issues after taking my car to the GM steeler for A/C work. Car went to them perfect and came back bucking and stumbling. Check the throttle. Put a scanner on and press your gas pedal to the floor and see if it shows 100% throttle. When I did mine it was only showing 57% which messed everything up. Sometimes you need to adjust the throttle cable after pulling the plenum. That fixed my problem. Good luck and keep us posted.

[2ljd]

Originally Posted by Paul Workman

Mine is a discontinued AutoXray unit - for which I also was able to purchase the computer program which produces graphs and charts from the data registered in the unit.

I believe AutoXray is "no more". I also purchased a Snap-On MT 2500 "in case" the AX dies. However, I need to also buy the appropriate cartridges to read OBD-1 duties.

Cool, thanks!

[Paul Workman]

Originally Posted by Billy Mild

I did some fuel pressure testing today. I bought an Actron Fuel pressure gauge. My results were FP1 static pressure with key on for a few seconds 44 PSI, FP2 with key on for a few seconds was 48 PSI on the dot. FP1 spiked to around 50 PSI then dropped down to 44 PSI. FP2 went to almost 52 PSI then dropped to 48 PSI. Would a clogged fuel filter cause a pressure difference?

I did the idle test with just FP1 and the pressure was right at 40-41 PSI.

It seems to bleed off after 10 or so seconds. The FSM says this is normal. I would still like to do the test for FPR test by jumping the 12v fuel pump test leads. I don't know where that jumper lead is located.

TL;DR- FP1 44 PSI, FP2 48 PSI for static fuel pressure test.

Is 44 PSI okay or should I go all anal and replace the fuel pump due to it not meeting the spec in the FSM?

Fuel presure...

Under static (engine OFF) there should be no fuel flow, normally. So, even if a filter is severely blocked, static pressure will not be affected unless there is a total blockage - in which case the motor would not run at all, leave-alone at WOT.

Except for the bleeding off (so rapidly) the (static) pressure reading doesn't look out of line, after a couple seconds, after the key is turned to ON, engine OFF. Mine reads 43-44, and that seems to jive with most others when operating well.

After turning the key OFF, the pressure (I've read) should hold for several minutes, if all is well. Mine holds w/in 1# of when I turned the key off for over 10 minutes (long enough to see it wasn't leaking!).

Bleeding off after only 10 seconds...says to me either an injector(s) are leaking or the check valve in one or both of the pumps is leaking, OR the hose clamp in the tank is leaking. (I installed the worm (screw) type hose clamps on mine. Been good for 5 years now. I doubt this has anything to do with cutting out/jerking/misfire (aka "bucking"???) but I would schedule it for a "fix", IF I didn't seek to do so "on the spot".)

Dynamic fuel pressure test:

When fuel is flowing, AND should the filter be partially blocked, then pressure will suffer due to a bad, partially blocked filter. BTW, I replaced mine at 42xxx miles and was amazed at the amount of crud that came out of that filter. It is cheap and easy to change, and like an air filter you should just do it periodically, IMO.

Taping the fuel pressure gauge to the windshield and going for a drive (dynamic test):



Cruising at 40 mph, the FP is hovering around 42-46#. Then, going WOT, the pressure jumps up to 53-55 for the duration of the WOT event, dropping back down to mid 40s again.

O2s...:

The O2 sensor output voltage continually cycles between high and low voltage, according to the exhaust gas pulses from each cylinder passing over it - or should at least. What you should be looking for are the "cross-count" numbers; the # of times the voltage reading switches from high to low. I don't have the exact figure, but it suffices to say they should be more or less symmetrical from left to right; also the BLMs should be "about equal" as well.

A "lazy O2 sensor will have significantly fewer "cross-counts, or the average will be significantly lower than than "normal" (if the average is hovering around the 300 or lower mV, the O2 is not working well. BTW, don't seal the wire harness where it enters the O2 sensor "for good measure". The sensor depends on atmospheric air sampling to make comparisons. IF dirt/grease or RTV sealant is closing off the opening to the atmosphere, the O2 can't work properly...ask me how I know.

The fact that it seems to run OK at WOT rules out stuff like the cam or crank position sensors. But, I'd want to rule out the MAP sensor. What does you scanner say re: the MAP readings?

Also, the Throttle Position Switch (TPS): It should be set at .5 VDC when closed. And, with the switch ON, engine OFF, the reading should increase smoothly through its entire arc - no drop outs! BTW, it isn't necessary to remove the TPS when removing the plenum. Simply lay the plenum on its back over the drivers' side (tire) w/ the throttle cables intact.

Spark plug reading:

Think of the spark plugs as data recorders: They will say volumes about fuel mix and ignition irregularities, if the misfiring is limited to one or two cylinders. If there is no clues to be had there, then that says something too!
ECM:

ECM: I'm not ready to suggest (yet) that exchanging ECMs is warranted, but as the other things are eliminated (by test verification when ever practicable - NOT, NOT, NOT by "it can't be this-or-that b/c that has been replaced", then the ECM is (of course) a common element. But, it might be worth running down a loaner, should it comes to it. Just a thought...

Oh! And, before testing further, clear the codes by removing power to the ECM for about a minute (should do it). Then, after it has been running (and misbehaving!), read the new codes. Also, know too that often a failing part can result/cause other seemingly un-related faults to occur. A bad O2 sensor will result in the injector dwell time on one side to be off, BLMs are affected by fuel delivery errors due to a bad O2 sensor...that kind of thing one has to keep in mind...

Frustrating, I know. But, too, as the circle spirals down, the issue is going to run out of hiding places. And, think of all the causal-related data you are learning along the way!

[Billy Mild]

Where can I get an extension like shown in the pic for my FP gauge?

TPS was replaced earlier this year by Marc Haibeck and is set to .54 volts key on and engine off. Fuel filter was replaced 2 years ago.

The passenger side O2 sensor has an extension harness on it and had black wire loom on it that ended up getting melted. The wires were fine just the wire loom was melted. I put new wireloom on the wire as well as put heatshielding tape around it to prevent that from happening again.

I will throw the scanner on it again and check the CrossCounts. Those appears to be inline maybe 0 sometimes 1 for each side.

There is another ZR-1'er who has a NOS computer that he will let me borrow to rule out the ECU. If this was a 924/944 and I was having this issue I would point to FPR, and/or DME cold solder joints going bad.

[Paul Workman]

Originally Posted by Billy Mild

Where can I get an extension like shown in the pic for my FP gauge?

TPS was replaced earlier this year by Marc Haibeck and is set to .54 volts key on and engine off. Fuel filter was replaced 2 years ago.

The passenger side O2 sensor has an extension harness on it and had black wire loom on it that ended up getting melted. The wires were fine just the wire loom was melted. I put new wireloom on the wire as well as put heatshielding tape around it to prevent that from happening again.

I will throw the scanner on it again and check the CrossCounts. Those appears to be inline maybe 0 sometimes 1 for each side.

There is another ZR-1'er who has a NOS computer that he will let me borrow to rule out the ECU. If this was a 924/944 and I was having this issue I would point to FPR, and/or DME cold solder joints going bad.

Gotcha.

The cross-counts...I'll have to do some more research/comparison. But, zero and 1 are waaaay low - ordinarily. I can't rule out a test equipment data error (yet). I'll have to get back to ya on that.

The extension I got at Harbor Freight, believe it or not. But, I've seen them on ebay and AutoZone's site (once) too.

Can the MT 2500 be "que'd" so that it can capture events in a time frame? My datalogger will do that and is how I was able to zero in on the the intermittent cause by seeing the "before and after" data when the problem occurred.

Are we having fun yet???