Wiped Cam Lobe? What are my options? (Page 1/7)
tnkgnr MAY 04, 11:18 AM
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.
theogre MAY 04, 12:04 PM
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)

The Ogre's Fiero Cave

[This message has been edited by theogre (edited 05-04-2021).]

pmbrunelle MAY 04, 12:13 PM
You're sure it's the cam lobe?

Check the rocker arm pivot balls for wear, myself and another person have had trouble with the Comp Cams Magnum rocker arms.
fishsticks MAY 04, 12:58 PM
Measuring for cam lobe wear should be done at the lifter itself, not the rocker.

As stated, check your rockers for proper adjustment and ball wear.

Pull the intake and measure lift the the lifter bodies to get a true reading.
tnkgnr MAY 05, 09:02 AM

quote
Originally posted by theogre:

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.




Hmm...drained the oil, which had about 1,000 miles on it and a lot of idle time while troubleshooting.

Just had these two shards after moving the magnet throughout the oil twice (2nd time no metal).
tnkgnr MAY 05, 09:08 AM

quote
Originally posted by pmbrunelle:

You're sure it's the cam lobe?

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.
fishsticks MAY 05, 11:18 AM

quote
Originally posted by tnkgnr:


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.

How does the car do at higher RPM/load?
tnkgnr MAY 05, 06:11 PM

quote
Originally posted by fishsticks:


Just throwing this out there.

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).
La fiera MAY 05, 06:58 PM

quote
Originally posted by theogre:

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!
V8Steve MAY 05, 07:24 PM
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.

Good luck.