Well - put a new camshaft and lifters in; Comp Cams 260H. Using a caliper on the old one, the lobes varied between 38mm and 40mm so it seems there was some wear. Car pulls even better than it did before on acceleration, but I still have the bucking issue when trying to maintain a constant speed
What kind of shape is the ignition system in? ignition coil, distributor (including icm module, short coil-to-distributor wire and pickup coil and distributor itself, rotor, cap, spark plug wires, plugs etc.
What kind of shape is the ignition system in? ignition coil, distributor (including icm module, short coil-to-distributor wire and pickup coil and distributor itself, rotor, cap, spark plug wires, plugs etc.
Immaculate - new ignition coil that tests good, new distributor (included pickup coil/rotor/cap/ICM), new plug wires, new plugs gapped to .045. Every sensor reads as it should. All motor mounts are fresh.
I wonder if my lack of EGR be causing this somehow, considering my bucking issue only occurs during low-load situations.
OK great -- what about the small coil to dizzy wire? Its right next to the exhaust manifold and is a common point of failure due to the heat. Maybe someone who knows more can chime in but perhaps the connection becomes erratic when it heats up and leads to a sputtering ignition system. You can test by taking it out, and hooking up a multimeter set to amps to each end. twist and bend the wire, if the ohms stay at no resistance the wire is fine but if it fluctuates then you know the cable intermittently has continuity problems. Looks like youve covered anything else so this last check should rule out the entire ignition system
OK great -- what about the small coil to dizzy wire? Its right next to the exhaust manifold and is a common point of failure due to the heat. Maybe someone who knows more can chime in but perhaps the connection becomes erratic when it heats up and leads to a sputtering ignition system. You can test by taking it out, and hooking up a multimeter set to amps to each end. twist and bend the wire, if the ohms stay at no resistance the wire is fine but if it fluctuates then you know the cable intermittently has continuity problems. Looks like youve covered anything else so this last check should rule out the entire ignition system
Have you looked at the fuel filter? or clogged cat?
Yea, that stubby wire came with the new plug wires, but confirmed the very low resistance across it with an ohm meter just now in a variety of different bendy ways.
Fuel filter is pretty new - only a few hundred miles on it. But if it was an issue, my problems would extend to points when I need more fuel, but my acceleration is great, it's the low-load situation that haunts this car.
Cat should be good (about three years old). I've had a clogged one before and this isn't it.
Did it have the problem before the removal of the EGR ? If I remember correctly doesn’t the EGR operate in the problem areas you described…
When I got the car it had no EGR and there was no issue. I added the EGR system back to the car (and got rid of my code 32). At some point the solenoid went bad, so just disconnected/plugged the system. I'm not sure if the two are related since it's not clear when the solenoid went bad.
Last idea I could possibly think of is the idle adjustment screw is out of whack. When the throttle is open it's getting plenty of air so there are no problems. But when the throttle is closed/cracked open, there is enough of a discrepancy between actual and commanded airflow to cause hesitation and jerking. The idle adjustment screw shouldn't be adjusted ever as far as I know but there is a procedure to set it back to factory setting. Should be easy to find with a Google search. Even if it doesn't fix your problem it will at least rule one more thing out
Sounds like you have a lean misfire on one cylinder. It will only show up at idle or cruise. It is small enough that the other cylinders are being commanded richer and compensating for the extra oxygen in the exhaust w/o tripping a check engine light.
Likely causes: Partially clogged injector. Intake gasket leak letting making a single cylinder lean.
Well - put a new camshaft and lifters in; Comp Cams 260H. Using a caliper on the old one, the lobes varied between 38mm and 40mm so it seems there was some wear. Car pulls even better than it did before on acceleration, but I still have the bucking issue when trying to maintain a constant speed
Any ideas left out there?
Hi! Hope I'm not too late... but wanted to make a few comments...
1 - OMG... "Antique" is that how old the Fiero is? I'm beside myself.
2 - Can we now confirm that the crank was not the concern?
3 - I don't know if this helps... but I had massive bucking one day with my Fiero, several years ago. I disconnected my MAP sensor, and the car was able to drive just fine. Acceleration wasn't too bad, but any time I tried to maintain speed, the car bucked like crazy. It just happened one day, and was really bad. I was able to get home by disconnecting my MAP sensor, and I ended up replacing it with a newer one: https://www.pontiacperforma...les/MAP_Upgrade.html
I can't say if that will solve your problem, but that is exactly the same symptoms I had, and replacing the MAP sensor solved it. I seem to recall that the MAP was also oozing fluid ??? which I didn't even know was a thing... but it didn't look very good.
Sounds like you have a lean misfire on one cylinder. It will only show up at idle or cruise. It is small enough that the other cylinders are being commanded richer and compensating for the extra oxygen in the exhaust w/o tripping a check engine light.
Likely causes: Partially clogged injector. Intake gasket leak letting making a single cylinder lean.
Idle works well - only a cruise problem. I don't think there's a clogged injector though. This issue existed on a previous set of injectors and I have a whole new (unused) set in there now with the same issue. Is there a way to confirm that the injectors themselves are receiving the electrical pulses to fire? Maybe a bad wire somewhere?
Originally posted by 82-T/A [At Work]: Hi! Hope I'm not too late... but wanted to make a few comments...
1 - OMG... "Antique" is that how old the Fiero is? I'm beside myself.
2 - Can we now confirm that the crank was not the concern?
3 - I don't know if this helps... but I had massive bucking one day with my Fiero, several years ago. I disconnected my MAP sensor, and the car was able to drive just fine. Acceleration wasn't too bad, but any time I tried to maintain speed, the car bucked like crazy. It just happened one day, and was really bad. I was able to get home by disconnecting my MAP sensor, and I ended up replacing it with a newer one: https://www.pontiacperforma...les/MAP_Upgrade.html
I can't say if that will solve your problem, but that is exactly the same symptoms I had, and replacing the MAP sensor solved it. I seem to recall that the MAP was also oozing fluid ??? which I didn't even know was a thing... but it didn't look very good.
1 - Not only is it an antique - it's been one for over a decade now!
2 - What issues could I expect with the crankshaft?
3 - Yea, I tried disconnecting the MAP, still had the same issue. I tested my old one just fine with a vacuum pump but ended up ordering a new one just in case. No luck.
1 - Not only is it an antique - it's been one for over a decade now!
2 - What issues could I expect with the crankshaft?
3 - Yea, I tried disconnecting the MAP, still had the same issue. I tested my old one just fine with a vacuum pump but ended up ordering a new one just in case. No luck.
Geeze, that sucks... really thought that would work.
Let me confirm some statements...
It stumbles when you try to maintain speed, correct?
Does it stumble on acceleration as well? Or only while maintaining speed?
My first thought (since it's not the MAP), is some kind of fuel issue...
Has it always done this since you've owned it, or is it something that just magically started one day?
Another thing could be the gap of the spark plugs (.35 vs .45), but if you haven't changed them in a while, then likely not that either... and that's a long shot.
It stumbles when you try to maintain speed, correct?
Correct - only when maintaining a speed; roughly in the 2k RPM area.
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Does it stumble on acceleration as well? Or only while maintaining speed?
No, it accelerates great. Idles solid. Deceleration through the RPM range is fine.
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My first thought (since it's not the MAP), is some kind of fuel issue...
Fuel pressure is good. Fuel pump is only a few months old. Filter is pretty young too.
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Has it always done this since you've owned it, or is it something that just magically started one day?
Just showed up one day - magic!
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Another thing could be the gap of the spark plugs (.35 vs .45), but if you haven't changed them in a while, then likely not that either... and that's a long shot.
Ugh... well, most of those responses sound good... so it sounds like at wide open throttle and decent throttle, it accelerates well... so there's no mechanical issues with things like the cam, or injectors, or something of that such.
It seems like it's possibly a throttle problem.
I didn't see in the other threads, but have you looked at the Throttle Position Sensor / TPS?
With a functional TPS and a functional IAC (Idle Air Control) valve, the car will idle well, and accelerate well at WOT. But... as you know, the TPS is basically a pentameter... in 2005+ cars that use drive by wire, the pentameter is moved from the TPS to the gas pedal, and eliminates the throttle cable and instead controls a solenoid.
Anyway, it's been over a decade (my Fiero is literally sitting in a storage unit where it's been for 12 years), but there is a little arm that needs to be adjusted on the TPS, which allows you to fine tune and adjust the TPS. At the same time... the TPS may be failing in that the sweep of the throttle is not providing the voltage it should in the lower RPMs. Since most of your driving is not wide open throttle, and most of your driving is going to be in the ~2000 rpm range... the contacts for the TPS will wear much more frequently in that range, which can often cause problems there.
You could either try adjusting the little tab to increase or decrease the amount of throttle your TPS is reporting, or replace the TPS and see if that fixes the problem.
:/ sorry I couldn't be of more help... but definitely interested in helping you get this fixed.
Originally posted by 82-T/A [At Work]: Ugh... well, most of those responses sound good... so it sounds like at wide open throttle and decent throttle, it accelerates well... so there's no mechanical issues with things like the cam, or injectors, or something of that such.
It seems like it's possibly a throttle problem.
I didn't see in the other threads, but have you looked at the Throttle Position Sensor / TPS?
With a functional TPS and a functional IAC (Idle Air Control) valve, the car will idle well, and accelerate well at WOT. But... as you know, the TPS is basically a pentameter... in 2005+ cars that use drive by wire, the pentameter is moved from the TPS to the gas pedal, and eliminates the throttle cable and instead controls a solenoid.
Anyway, it's been over a decade (my Fiero is literally sitting in a storage unit where it's been for 12 years), but there is a little arm that needs to be adjusted on the TPS, which allows you to fine tune and adjust the TPS. At the same time... the TPS may be failing in that the sweep of the throttle is not providing the voltage it should in the lower RPMs. Since most of your driving is not wide open throttle, and most of your driving is going to be in the ~2000 rpm range... the contacts for the TPS will wear much more frequently in that range, which can often cause problems there.
You could either try adjusting the little tab to increase or decrease the amount of throttle your TPS is reporting, or replace the TPS and see if that fixes the problem.
:/ sorry I couldn't be of more help... but definitely interested in helping you get this fixed.
Really appreciate your interest in helping me get this fixed - I'm completely stumped...
For the TPS, I did monitor the voltage through the sweep of throttle, and it looked good (both the voltage range and no hiccups in the area that would normally wear). Installed a new one anyways just in case. Didn't change anything. I don't think I've tried with it unplugged though, so I'll give that a run.
For the throttle itself, it had probably been molested at some point, since the screw cap was missing. But I went through the procedure to ensure the proper adjustment (hopefully correctly). It idles at or a hair under 1k rpm.
Perhaps one clue I should mention - the bucking doesn't happen for the first 2 or 3 minutes of driving. I've tried disconnecting the O-2 sensor to see if there was some sort of relationship there, but that was a dead end too.
I recently replaced my oil pressure sender because it was reading erratically. Turns out it was still the original one and was completely corroded inside. While researching it I learnt the oil pressure sender also feeds the fuel pump in parallel with the fuel pump relay, in case the relay fails the fuel pump will still run off the oil pressure sender circuit. I mention this because my sender would read fine during the first few minutes of driving but after it got warmed up it would read far lower then the actual oil pressure (confirmed with mechanical gauge).
Im not sure if this would even be possible but maybe your fuel pressure relay is dead and the fuel pump is being run solely off the oil pressure sender. After driving your car a few minutes your failing oil pressure sender starts reading abnormally and sends sporadic voltage to the fuel pump, leading to your jerking motion. But, this doesnt explain how your issue manifests only at constant speed, im not sure how the oil pressure sender even if it were failing could affect that. But, you could test if the fuel pump relay is working by listening for it clicking on when you turn the key to 'on' and the pump primes for 3 seconds. the relay is mounted at the top left of the firewall above the air filter. you could also just unplug the oil pressure sender, if the jerking persists it obviously isnt the sender. It only feeds the gauge so it wont effect anything wont even throw a CEL
Do I dare say I figured it out? Unplugged the tach filter after reading elsewhere that problems with it can result in "hiccups" and other issues. Drove it around and seems fine. I'm skeptical though - why would it have such a huge impact? My tachometer was working, so I dunno...but tentatively seems okay.
Pic of my ride, cause I'm in love with this wrap job
I have those too, and with my old engine, I started having issues. I was considering re-using these in my new 3.4, but now questioning if I should even bother. What kind of issues did you have? Were they seeing premature wear? Nothing re-adjusting the valve lash couldn't fix?
I have those too, and with my old engine, I started having issues. I was considering re-using these in my new 3.4, but now questioning if I should even bother. What kind of issues did you have? Were they seeing premature wear? Nothing re-adjusting the valve lash couldn't fix?
Thanks!
My experience is with the 1.52 ratio version of the 1.60 rocker arms you linked to (Comp Cams Magnum).
Summary: I destroyed two sets of Comp Cams Magnum rockers. (burned up, shredded pivot balls) Comp Cams Magnum rockers appear to have a flawed oiling design. Stamped Melling stock replacement rockers lasted me about 10000 km (much more reliable than Comp Cams). It seems like the performance valve springs and installed height on the short side caused valve opening forces on the high side, which didn't help anything. I'm running Scorpion roller rocker arms now, and all seems OK.
In order of preference / perceived reliability: 1. Scorpion roller rocker 2. Stamped rocker arms such as Melling 3. Comp Cams Magnum
If your 3.4L longblock comes with stamped rocker arms, I think you might as well use them. If they don't suffice, then it might be worth the trouble to switch over to Scorpion.
I would avoid Comp Cams Magnum.
Edit to add: If you're installing a high-lift cam with stiffer valve springs, you might want to lean towards running the Scorpion rocker arms. Note that some supporting hardware is needed to install Scorpion rockers, besides the rocker arms themselves.
[This message has been edited by pmbrunelle (edited 05-12-2023).]
Summary: I destroyed two sets of Comp Cams Magnum rockers. (burned up, shredded pivot balls) Comp Cams Magnum rockers appear to have a flawed oiling design. Stamped Melling stock replacement rockers lasted me about 10000 km (much more reliable than Comp Cams). It seems like the performance valve springs and installed height on the short side caused valve opening forces on the high side, which didn't help anything. I'm running Scorpion roller rocker arms now, and all seems OK.
In order of preference / perceived reliability: 1. Scorpion roller rocker 2. Stamped rocker arms such as Melling 3. Comp Cams Magnum
If your 3.4L longblock comes with stamped rocker arms, I think you might as well use them. If they don't suffice, then it might be worth the trouble to switch over to Scorpion.
I would avoid Comp Cams Magnum.
Edit to add: If you're installing a high-lift cam with stiffer valve springs, you might want to lean towards running the Scorpion rocker arms. Note that some supporting hardware is needed to install Scorpion rockers, besides the rocker arms themselves.
Thanks PM... I appreciate it. I'll look into this. The most telling thing I noticed was the fact that the oil passages on the Comp rocker arms are not angled towards the fulcrum... really poor design there. I wonder why they did that.
Was going to guess O2 sensor or vacuum leak of some kind, also check grounds to motor, provides ground for all the sensors. Bad MAP or TPS also possible to cause these issues. Most other issues would cause WOT issues.
