Wiped Cam Lobe? What are my options? (Page 2/7)
tnkgnr MAY 06, 11:32 AM

quote
Originally posted by La fiera:

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
pmbrunelle MAY 06, 12:46 PM

quote
Originally posted by tnkgnr:


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?
tnkgnr MAY 06, 01:14 PM

quote
Originally posted by pmbrunelle:

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.
Patrick MAY 06, 03:49 PM

quote
Originally posted by tnkgnr:

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).]

pmbrunelle MAY 06, 07:25 PM
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.
fishsticks MAY 06, 08:55 PM

quote
Originally posted by Patrick:

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.
La fiera MAY 06, 11:30 PM
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.
tnkgnr MAY 07, 11:54 AM

quote
Originally posted by Patrick:

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).]

tnkgnr MAY 14, 07:33 PM
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?
Raydar MAY 19, 08:07 PM

quote
Originally posted by tnkgnr:

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.