Hey all - I have an 86 3.4l pr with a 5 speed and have been chasing a stumbling/bucking problem in low-load conditions (engine bucks when I'm trying to maintain a speed in the correct gear - ie 2nd at 20 mph or 5th in 65 on level road). All my sensors read correct and I've blocked off the EGR. Timing is at 10 degrees (yes with jumper) and bumping that up or down didn't change the problem. Idles and accelerates just fine. Has 1.52 roller tip rockers.
Today I took my valve covers off and measured the travel of the pushrods as they pushed on the rocker while manually cranking the engine. On the 1/3/5 bank I saw the exhaust get pushed up 8/8/10mm respectively and on the intake I saw 8/7/5mm respectively. I presume the lobe on the #5 is worn on the intake?
Assuming I've confirmed a worn cam lobe, the only remedy is to drop the cradle and put a new cam/lifters in, correct? Of course if I'm dropping the engine I'd probably need to go ahead and do new pistons and timing chain as well...sounds like a nightmare...which makes me wonder if I should get a rebuild kit or just spring for an ATK refurbished engine.
Hydro lifters often won't pressurize when cranking. So Don't trust any/all lifters to get same height cranking the engine.
Oil Pump has nothing to do w/ lifter pressure, only mainly keeps lifters "drowning" in oil, in this type setup. Some Other engines need full pump pressure to work right and you never get that cranking the engine.
If the rockers are not adjusted right, can screw up the lifters too when measuring lifter height. Can be why the Cam died too if cam is real bad.
You should have a lot of metal in the oil pan and cam valley. Can drain the pan and simple look at oil or send to lab to test the oil.
Fix? After cleaning up the mess, you replace the cam and lifters at minimum but likely need cam bearings. Cam bearing must be install right or cam won't last long.
------------------ Dr. Ian Malcolm: Yeah, but your scientists were so preoccupied with whether or not they could, they didn't stop to think if they should. (Jurassic Park)
Check the rocker arm pivot balls for wear, myself and another person have had trouble with the Comp Cams Magnum rocker arms.
I'm not sure of anything. More like I've gone through everything else, and camshaft seems to be the final item. I took two of the rockers off today and the pivot balls both are smooth without deformation - I'll take the rest off tomorrow.
I'm not sure of anything. More like I've gone through everything else, and camshaft seems to be the final item. I took two of the rockers off today and the pivot balls both are smooth without deformation - I'll take the rest off tomorrow.
Just throwing this out there.
Clogged fuel injectors can cause low speed issues. The spray pattern is all messed up and so atomization sucks.
Clogged fuel injectors can cause low speed issues. The spray pattern is all messed up and so atomization sucks.
How does the car do at higher RPM/load?
Pulls well under load, accelerates nice. For example in my 20mph bucking in 2nd, I can make it go away if I go to 4th. The real drivability problem is highway where there are no more gears to go to! (so I found myself accelerating to 70, foot off the gas to 65, accelerate to 70)
I've got brand new unused injectors and I've got good fuel pressure (tested via the rail, and the fuel pump/filter is new).
Hydro lifters often won't pressurize when cranking. So Don't trust any/all lifters to get same height cranking the engine.
I Igree. If you engine is idling fine and acting fine except at low gears then the problem is in the calibration (timing/EFI) not mechanical. If you had a worned lobe you'll have a miss at idle and troughout the entire rpm range. This is not your case. Check your MAP sensor signal and make sure you have no vacuum leaks. Vacuum leaks wil be more noticible at higher rpms and under light loads (higher vaccum). That would through the AFR's out and make the engine stumble and miss. Good luck mate!
I've had a similar experience with my 355 SBC and Holley multiport injection. Since a few comments point to the fuel here's my experience of a few weeks ago:
First, at light load, partial throttle, 1800-2000 RPM on a slight grade there was an occasional bucking. Second, on the highway around 3000-3200 RPM & 60-65 MPH I would get similar occasional bucking, more like skipping a beat. When that happened I'd either accelerate or reduce speed to get out of that zone.
Other than the above two situations the car accelerated fine with no issues. At idle there was an occasional hiccup as well and I changed the AF ratio today.
This car had been remotely tuned by a nationally recognized individual around October of last year but I hadn't driven it much. So in April, we talked about this bucking problem. A few weeks ago we made an appointment to live tune it on the road by using my phone as a hotspot to power the laptop internet so that he could then take over while I drove. His very first observation was that the learn table showed issues in the two troublesome zones and that the AF ratio was too lean. After shutting down the car, clearing the table, adjusting and blending the AF ratio in these areas, "we" went for drive. The first portion was at light throttle up the local hills in various gears and various rpms, some with more lugging that others. Then it was off to the interstate to hold the rpm in the bucking range where I could always reproduce it. RESULTS: All problems gone. The entire session was less than a half hour.
So now I've applied the same reasoning to the idle zone in the learn table where it sort of "skips a beat" and will check it in the coming days.
I hope this helps you out. BTW, when I mentioned my problem to another tuner who had previously installed the EFI and done a chassis dyno....he immediately suspected the bucking was related to a lean mixture or timing.
I agree. If you engine is idling fine and acting fine except at low gears then the problem is in the calibration (timing/EFI) not mechanical. If you had a worn lobe you'll have a miss at idle and throughout the entire rpm range. This is not your case. Check your MAP sensor signal and make sure you have no vacuum leaks. Vacuum leaks will be more noticeable at higher rpms and under light loads (higher vacuum). That would through the AFR's out and make the engine stumble and miss. Good luck mate!
Thanks - I need it! I tried bumping the timing to about 5 degrees and then to about 14, which didn't change the problem. I tested the MAP sensor with a vacuum gauge and also looked at it through ALDL and it seems fine (gauge itself held a vacuum and appropriately changed voltage as I applied more). I'll order up a new set of intake gaskets and double-check all the lines.
Would a worn timing chain present these problems? I've also had some mild popping when going downhill, foot off the gas and in gear.
Here's a chart showing the MAP data vs. Speed in a test drive
Thanks - I need it! I tried bumping the timing to about 5 degrees and then to about 14, which didn't change the problem. I tested the MAP sensor with a vacuum gauge and also looked at it through ALDL and it seems fine (gauge itself held a vacuum and appropriately changed voltage as I applied more). I'll order up a new set of intake gaskets and double-check all the lines.
Would a worn timing chain present these problems? I've also had some mild popping when going downhill, foot off the gas and in gear.
Here's a chart showing the MAP data vs. Speed in a test drive
I don't know if a worn timing chain would cause these problems, but Fiero timing chains do stretch out pretty quickly.
In that datalog, is the MAP going up and down because you're pressing/releasing the throttle pedal, or your foot is constant and the MAP is doing that on its own?
I don't know if a worn timing chain would cause these problems, but Fiero timing chains do stretch out pretty quickly.
In that datalog, is the MAP going up and down because you're pressing/releasing the throttle pedal, or your foot is constant and the MAP is doing that on its own?
Probably me - whenever it bucks I'll either get my foot off or punch it harder, otherwise it seems to get worse. TPS data added.
I tried bumping the timing to about 5 degrees and then to about 14, which didn't change the problem.
Keep in mind that the timing scale is only relevant if the outer ring of the harmonic balancer (where the timing marks are) hasn't spun.
Considering all the trouble you're having, it would be prudent to check and make sure that TDC of #1 cylinder actually corresponds with 0° on the scale.
[This message has been edited by Patrick (edited 05-06-2021).]
The bucking back and forth with light throttle (neither accelerating nor decelerating) almost sounds like the engine might be moving around on damaged trans/engine mounts.
Keep in mind that the timing scale is only relevant if the outer ring of the harmonic balancer (where the timing marks are) hasn't spun.
Considering all the trouble you're having, it would be prudent to check and make sure that TDC of #1 cylinder actually corresponds up with 0° on the scale.
This.
Timing decreases as load increases, so if you're overly advanced at a light throttle it can clear up as engine load increases.
Lean areas in your fuel map like Steve said are also possible, but less likely unless you have a custom tune or a vacuum leak.
Do you have the coil mounted on the stock location? When was the last time you changed the cap, rotor (if it's non DIS), wires and spark plugs?? Start with the basics and then if problem persist move on.
Keep in mind that the timing scale is only relevant if the outer ring of the harmonic balancer (where the timing marks are) hasn't spun.
Considering all the trouble you're having, it would be prudent to check and make sure that TDC of #1 cylinder actually corresponds with 0° on the scale.
I checked this just now. On the compression stroke and using a scope to see, the piston completely stops its upwards travel when the thick line of the balancer is about the 0 degree mark and then rotates a few more degrees before the piston starts going down.
Ignition components are all new and mounted in the stock location/configuration - wires, plugs gapped to .045, cap, button, distributor, and coil.
Mounts are good - new ones (less than 500 miles) all around and poly on the dogbone.
Fuel components are all new - injectors, fuel pressure regulator, pump, filter and showing appropriate pressure.
Coolant sensor, MAT, TPS, IAC, MAP all appear to have correct values.
Emissions - EGR is disconnected and blocked off. O-2 sensor is new.
[This message has been edited by tnkgnr (edited 05-07-2021).]
New upper and lower intake gaskets on and everything buttoned back up. Ensured my vacuum lines are good.
I keep thinking there must be something mechanical that's wrong. When I crank the engine by hand, I can get about 6 degrees of travel before the rotor on the distributor starts to move. Would a stretched/worn timing chain cause my problems? Any other ideas?
New upper and lower intake gaskets on and everything buttoned back up. Ensured my vacuum lines are good.
I keep thinking there must be something mechanical that's wrong. When I crank the engine by hand, I can get about 6 degrees of travel before the rotor on the distributor starts to move. Would a stretched/worn timing chain cause my problems? Any other ideas?
A stretched or worn timing chain will affect not only spark timing, but cam timing, since the distributor is driven off of the "back end" of the cam. I can't say that it would cause your problems, but it certainly wouldn't help.
The best time to check for timing chain slack was when the lower intake was off.
Pull a valve cover and watch the rocker of a valve that's opening/closing. Turn the crank both directions and make sure the rocker moves without any delay.
Took the timing chain cover off. Chain didn't look too stretched, but replaced it and the sprockets anyways.
It does appear that things might be 180 degrees off though. When the sprocket dots align, the distributor is pointing to the #4 wire like this:
When its at #1 TDC on the compression stroke, it looks like this:
Should I go ahead and rotate the cam around so the dots point at each other at #1 TDC, or does it even matter?
The camshaft rotates at exactly half the crankshaft speed. So if you line the dots up in one direction, it will line up the other way also if you spin the motor over.
I would put it dot to dot and set your distributor to #4. Pull the plug on #4 to make sure it's at TDC. Also check and make sure both valves are closed.
[This message has been edited by fishsticks (edited 06-02-2021).]
Did you get a chance to try anything else for this problem? It seems like a real heart breaker
One step at a time - will definitely post whatever the problem was once it's found (and whether or not changing the chain did anything). Though given the condition of the timing chain/sprockets, I doubt this was it.
I would put it dot to dot and set your distributor to #4. Pull the plug on #4 to make sure it's at TDC. Also check and make sure both valves are closed.
Will air whooshing out of the #4 spark plug hole confirm its valves are closed, or does this require removing the valve covers again?
Will air whooshing out of the #4 spark plug hole confirm its valves are closed, or does this require removing the valve covers again?
Put your finger over the spark plug hole. If the motor tries to blow your finger off the hole, you're on the compression stroke, which is where you want to be.
All put back together and test drive complete. There's less bouncing back and forth on the harmonic balancer when timing, but the issue remains...sigh...
I did notice something weird though. When I went to do timing I noticed that in diagnostic mode with the engine off, it appears it is quickly pulsing the injectors. Never done that before...
[This message has been edited by tnkgnr (edited 06-05-2021).]
All put back together and test drive complete. There's less bouncing back and forth on the harmonic balancer when timing, but the issue remains...sigh...
I did notice something weird though. When I went to do timing I noticed that in diagnostic mode with the engine off, it appears it is quickly pulsing the injectors. Never done that before...
Is the timing mark on the balancer jumping around when it's in diagnostic mode? I had a bad pick up coil in the distributor cause that before. And are you sure the noise with the engine off isn't the iac valve?
Is the timing mark on the balancer jumping around when it's in diagnostic mode? I had a bad pick up coil in the distributor cause that before. And are you sure the noise with the engine off isn't the iac valve?
Hah - yea apparently it is the IAC (confirmed by unplugging). Does that mean anything?
The balancer is pretty steady. When I do the timing I use the ignition coil wire so it's showing the #1 and #4 back to back. Between the two about 2 degrees difference.
Took things apart after figuring out how to get the lower intake off. The cam lobes appear to be fine (roughly measured the high point of each lobe by using a pencil), as well as the lifters.
However, I figured out my #6 pushrod is bent. Is this a good candidate for my symptoms? I suspect maybe caused from excessive pinging when I had different injectors in. Anything else I can check on while I've got everything opened up? Any way to check if that valve is jacked up? Thoughts?
Yes. Install the lifter, and new pusrod and rotate the engine and pay attention at how the lifter moves up and down compared to others. If it looks good zip her and you should be good to go.
Yes. Install the lifter, and new pusrod and rotate the engine and pay attention at how the lifter moves up and down compared to others. If it looks good zip her and you should be good to go.
Hmph...I installed the lifters and rotated the engine around. They all went up about 7mm as measured below (to include the one with the bent pushrod), except for the #5 intake valve lifter, which raised only 5mm.
[This message has been edited by tnkgnr (edited 01-02-2022).]
Well - buttoned it all back up with the new pushrod. Happy it cranked back up with minimal effort (the most I've taken an engine down before), but still have the same bucking. Assume the 2mm less lift on the #5 intake lifter is my issue and a new cam is necessary.
[This message has been edited by tnkgnr (edited 01-18-2022).]
Anyone have recommendations for a cam? I've got the 1.52 roller tip rockers and ported out the upper/mid/lower intakes (haven't touched the heads). I'm really just looking for something I can put in that maintains or improves the performance of the 3.4l without any additional expense (ie. machining the heads for better springs, etc).
And then the act of changing the camshaft is just getting the timing gear off, distributor off, lifters removed, and just pull it out? Replace with new lifters and copious amounts of assembly lube, correct?
