Sudden Power Loss

[Lawndart]

Scenario w/background:
Bronco I purchased a few years back with 37k on the odo (302). I have to assume 137k.
This has been a rolling refresh and running well - now displaying 67k. Most systems Inspected and Replaced As Necessary (IRAN).

A few weeks ago I noticed a "miss" which turned out to be a bad u-joint in rear shaft. I took this opportunity to do a spring tune-up: plug wires, points, condenser, plugs, timing, vac check. Been running very well since.

Incident:
Yesterday I was driving and came to a stop. Upon acceleration, I noticed a lack of power; almost like starting in 3rd or 2.5 gear. Circled back to the shop. Nothing improved or worsened. I was gentle.

Initial troubleshooting:
In park (auto tranny), I was idling at 500rpm; normally around 900.
Timing, re-timing, confirmed #1 at TDC per visual and per markings on balancer.
Checked timing chain slack - about 1/8 turn on wrench attached to crank.
Stopped.

Today's troubleshooting (more organized):
Without question, #1 TDC is properly indicated on balancer.
Re-stabbed distributor, initial setting 10 deg BTDC.
Will fire, but not start.
Swapped in old condenser - no change
Swapped in old points (now with old condenser) - no change.
If I aggressively advance the timing and hold the pedal to the floor, I can maintain a poorly running engine.
Compression check on #1 reveals 75psi.
Compression check on #5 reveals 75psi.
Stopped.

I think my timing jumped a nylon tooth or two.
Thoughts???

 

[Joe473]

The jumped tooth is very possible. The compression test makes it seem that way. Below is a timing chain on a 140000 1971 original 302. Didn't jump but take a close look at the teeth. A few are just about gone.

Before going crazy though grab a dwell meter and check the points are properly set. Also double check you are not a tooth off on restabbing that distributor.

If you drive on a regular basis and the rest of the engine is good do the chain. Take your time with water pump and timing cover bolts.

Good luck.

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[DirtDonk]

That 1/8 turn is another giveaway for a dead or dying chain. Optimally you'd want only a few degrees of rotation on the crank (as measured with the timing marks) with a typically quoted end-of-life number of 10° of crank rotation.

I think brand new they still have about 4 or 5 degrees of rotation, and at 7 degrees they're worn but still usable. At 10 degrees or more of slop, it's time to consider changing it. If your measurements are correct, that's about 45 degrees of rotation. Correct?

How were you checking?
As far as you know the engine is original? What oil do you use in it?
Just wondering if some of the cam lobes are going flat as well, since that compression number is up to half of what you might expect. But again, what cam is in it I wonder.
And what year engine is this? Some compression numbers could have been as low as 95-110 pounds, but more often than not a healthy 302 might have 120 to 145 psi compression. An engine with aftermarket stuff and high compression stuff could be as high as 175, but depending on your year and if it's stock, just over 100 to 140 would not be out of line.

But 75 is bad in any 302 book. Did you perform the test like normal, with all plugs out and the carburetor throttle plates held wide open?

Good luck.

Paul

 

[Lawndart]

Hey Joe, that is what I expect to find inside. I did use a dwell meter when I installed the new points and confirmed again yesterday with the meter (around 24-25 deg, iirc). I was pretty deliberate about the stabbing, I'm thinking it was OK. Plus, it was running well, then poorly with no distributor movement. It was only a (cough) stab in the dark resetting the dist.

Hey Paul, I'd say the engine is basically stock - 1973. I pulled the original 2v intake (year correct) and installed a 4v 30k ago. The PO did install headers, but that is all as I see it. The alternator (proper for original) became noisy at one point, and I took it apart and replaced the bearings - they were FOMOCO. Valve covers correct, etc. Heater hoses are Motorcraft. So, I'd say virtually original.
I checked compression early on when I got the Bronco, they were 120s. I change oil every 3k miles with Rotella 15-40 in the summer and 10-30 in the winter with the addition of ZDDP at each change. I even mix a little avgas into my aux tank to give the engine a taste of lead every now and then. Although I do drive it a lot as a daily driver, it is not abused in any fashion. I did not remove all plugs for the compression test, just the two fronts; throttle was set at WOT.
Tomorrow, I will test compression with another gauge (just in case) and the timing chain measurement again (to be more precise).

Thanks so far.

 

[Joe473]

Do a leak down test to determine if ring or valve are bad and how bad before you start taking thinks apart. If leakdown is good then thats another sign it jumped and cam timing lowered cranking compression and there may be no other major issues. If you weren't burning oil you may get away with a timing chain and valve job if you're looking to keep it original. 120 psi is about where mine was at and not bad for that year. No smoke but mine burned a good amount of oil.


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[Joe473]

Also for good compression test remove all plugs, strong battery, warm engine if possible, and wide open throttle.

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[Broncobowsher]

Before you tear into the engine, go for a drive and make sure the transmission is shifting gears. It is possible the transmission is stuck in high gear.

 

[Lawndart]

Right now, it is not running well enough to drive and I fear any additional stress could do damage.
About the leak-down. I am familiar, but if the gears have slipped, wouldn't the valves be slightly open at TDC?
I am certain the tranny was shifting; I went to manual mode when it initially acted up. I commonly shift the tranny manually.

The change was sudden, I mean in the matter of feet, seconds... At this point, the engine does not want to run. Normally I could start it by just reaching inside and turning the key and it would start instantly, virtually no cranking.

The engine did not use oil or smoke - less that 1qt between changes, but some of that is drippings from the oil pan tube.

 

[Joe473]

Yes cam timing may have changed and can affect leakdown if you don't make sure valves are closed. If you are on compression stroke there is significant duration with valves closed. Considering it's running not that far off. You will need to watch rocker arms while turning over by hand to confirm valves are closed to do a proper leakdown. Or loosen rocker arms. Leakdown is not affected by cam timing if done properly.

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[DirtDonk]

Yep. Just go by the visuals and not the numbers on the damper anymore.

What about a big vacuum leak? That would definitely effect the running nature of things, but I don't think it would effect your compression readings or leak down results.

The sudden nature of this would indicate more involving the timing chain than the typical vacuum leak I would think.
But it doesn't hurt to look at things sideways sometimes.

Paul

 

[Broncobowsher]

Chain jump is sudden and usually still runs but very badly.

 

[Lawndart]

Broncobowsher - I'm thinking the same.

Just did a more precise slop test: 16-18 deg...

 

[DJs74]

Broncobowsher - I'm thinking the same.

Just did a more precise slop test: 16-18 deg...

I'd go ahead and pull both valve covers and have a look at the valvetrain since you are in the inspection mode. A rocker arm jumping off, a dead plug or a bad wire will feel the same as you are experiencing (all equate to being down a cylinder)

DJs74

 

[Lawndart]

Pulled the right valve cover today - all parts move and look kosher.
Maybe I'll pull lefty tomorrow, just for grins.

Immediate plans:
Order parts and proceed with timing chain replacement.
On the forum, someone mentioned damperdudes.net. I called them today and they will rebuild my existing damper in a-day-turn-around. I would reference the recommendation, but I have read too much and for the moment, it is lost in the ether.
I have found a chain/sprocket set made in US.
Hopefully no further damage has occurred.

I know the engine is a bit tired and at some late autumn in the future, I plan on pulling for a full overhaul. If I can return it to the same running fashion as a few days ago, that may not need to happen until this fall or next or...

Pics and progress as it happens.

Until then, my trusted ole F100 is being promoted for daily duty.

Thanks again.

 

[edmedlin]

Might be too obvious, but did you fill up at a different station than usual or a station that you never used before? I had very similar things happen to me when I got a bad tank of gas. I used some Lucas gas additive and top-end conditioner with a new fuel filter and it ran a lot better until I could get some good, 91 octane (no ethanol) into it. It continually got better until it ran normally. Just an idea that might save some time. It does sound like a timing issue, but bad gas can drive you crazy.

 

[Lawndart]

Thx - I thought about that a bit and changed over to my Aux tank. Same result.
The fuel was from my usual 91 E0 station.

All mental exercise at the moment as of now, I am missing a radiator, harmonic balancer and a water pump. The timing cover should reveal the demons later today.

 

[Lawndart]

I'll let the photos do most of the talking ;D

Looks like a tooth or two jump.

I'm guessing it was timing for a chainge

Anyway... does it matter that the new cam gear does not have the nice raised region for the fuel pump eccentric?

 

[DJs74]

I'll let the photos do most of the talking ;D

Looks like a tooth or two jump.

I'm guessing it was timing for a chainge

Anyway... does it matter that the new cam gear does not have the nice raised region for the fuel pump eccentric?

I'd say the Nylon cam gear has served its purpose and its time to be replaced.

No, shouldn't matter that the new steel gear doesn't have as much of a pronounced raised area for the fuel pump eccentric - you can take a quick measurement on the overall thickness of old vs. new just to confirm.

DJs74

 

[Broncobowsher]

There is the one and two piece fuel pump ecentric. They have different thicknesses and the thickness of the timing gear also varies. So long as they all match you are good.

 

[Lawndart]

I compared old to new gear. Dimensions are the same, including dish, thickness etc. I'll get them on today and start the reassembly.