1970s 1978 425 cid, 7.0L engine timing chain/gear failure rate

[J. Jackson]

Does the 1978 425 engines have timing chain/gear issues like the newer models?  According to the internet some models have chains failing at 40K miles.  I cannot imagine steel gears and chains failing that early unless the engine has an oiling issue.

[35-709]

The cam gear is/was plastic coated steel, in time the plastic fails.  For some reason some last longer than others but sooner or later they are destined to fail.  Maybe Bruce Reynolds will dig out his photos of the pieces he took out of his '72 Eldorado's oil pan that also pretty much plugged up his oil pump intake tube.  GM used these gears, supposedly with the thought that they were quieter, for many years.  Few, if any, here can hear the difference. 

[Eric DeVirgilis CLC# 8621]

I believe it's only the tips of the teeth that have the nylon coating, not the entire gear. Some owners seem absolutely terrified of this design feature which GM had been using for many years but having owned LOTS of 425 engine Cadillacs with mileages ranging from 10k to 200k I have yet to have a problem associated with it.

["Cadillac Kid" Greg Surfas 15364]

It is only an issue for "interference" (small chamber cylinder head) motors, 1968-1973 where a slipped timing chain usually leases to bent valves. For the later motors a cam gear failure only leads to a tow home. I too have seen motors with 100k that have not shed the cam gear nylon. But I have also seen (one that is currently my daily driver) that had to have the timing set changed at less than 50k
Greg Surfas

[V63]

I had a 1970 472 that failed at 100k, (OEM cam
Gear) of coarse 300 miles from home. went to pull away from a light and car simply stalled. Thought I was out of gas 🤷🏽�♂️. No valve interference.

Full glamour experience package,  Involved Towing, rental car, hotel and repair services and stranded a friend with me too.

[bcroe]

I had a 1970 472 that failed at 100k, of coarse 300 miles from home. went to pull away from a light and car simply stalled. Thought I was out of gas. No valve interference.
Full glamour experience package,  Involved Towing, rental car, hotel and repair services and stranded a friend with me too.

Yes you can choose the time to change the timing set,
or the car will choose it for you.  In the mean time,
the plastic wears down much faster, got another note
this month showing the slack, and stating the car
definitely ran better after the replacement. 

Under the plastic is aluminum, which wears badly,
but might keep the chain going for a while, if the
oil pickup is not blocked.  But a little more
running and the chain falls off. 

It is much cheaper to mold/dicast the OEM sprocket
than to machine steel.  And it means another reason
old cars will be replaced with new ones.  But like
so much logic promoted today, after the decision is
made, a nice sounding justification is found (quiet). 
Bruce Roe

[Eric DeVirgilis CLC# 8621]

Any number of other things can fail without warning, disabling the car leaving one stranded. Should every one of them be replaced too? Where does it start and where does it end?

[smokuspollutus]

I have had these plastic ones fall apart on me in different cars, but can't recall it ever happening on a big block Cadillac.

The valvetrain load on these engines is light and they never rev over 2500-3000rpm unless you're really flogging it.

I pulled a 39 year old 100k mile one in perfect condition out of my '83 (different engine). The plastic gear wore better than the cam it was attached to.

I would not preemptively change one unless you have walking timing or an oil leak bad enough to pull the cover. If it's really eating you to know, yank the distributor and drop a cheap boroscope down there and see what kind of shape it's in.

GM used these gears from the 60s through to the 80s as each engine family was updated with roller cams.

[J. Jackson]

What prompted the question is I have a miss at idle on my 1978 7.0L engine, not steady but random.  I have replaced the spark plugs, plug wires, dist. cap, HEI module and the miss still persists with engine hot or cold.  I've pretty much eliminated vacuum leaks using brake cleaner, propane and smoke test.  No leaks at intake manifold.  Cat converter has been removed and the car does have AIR pump but it's not a Calif. car.  Mileage is 87K.  I plan a compression test tomorrow and hook up my timing light to see if the timing jumps around. 

[Eric DeVirgilis CLC# 8621]

Sounds to me like a valve is not seating properly at certain points of its rotation.   

[J. Jackson]

Sounds to me like a valve is not seating properly at certain points of its rotation.   

That leads me toward the timing chain/gear.

[The Tassie Devil(le)]

Here is what happens to the Cam Gear when the nylon covers come off the teeth.

Notice that the timing chain is loose, but the engine still ran beautifully.  All those "bits" were removed from the oil pump pickup screen.

The timing will be erratic as the pieces fall off, but when I pulled my engine down, there was no way of telling when it happened, but the wearing to the aluminium, and the marks to the Crank gear showed that it had been a fair while before I purchased the car.

The other matter I have to report is that even though the oil pump intake screen was full of these pieces, there was no loss in Oil Pressure observed when I fitted the gauge.

Bruce.

[Eric DeVirgilis CLC# 8621]

That leads me toward the timing chain/gear.

That wouldn't have anything to do with a faulty valve.

[TJ Hopland]

Since you will have all the spark plugs out for the compression test that will make the engine easier to turn so with the distributor cap off rock the crank back and fourth and watch how long it takes the rotor to respond and change direction.  That will give some clues as to the timing sets condition.

You can see enough of it looking down the dizzy hole (if things are not super dirty) to get an idea if its an original and its condition.

My peak times messing with these cars was around 2000. Around that time I had a couple cars that were not especially high mile and had been sitting.  Looking down the dizzy hole on both of them I could see it was still the plastic gear but they both looked to be in reasonable condition an I didn't see any cracks. 

One of them was a 78 that I was pretty much daily driving and about 10k miles later which I think was around 100k total on the car it just started not feeling right.  Kinda chased a bunch of the other systems and didn't really find a problem till I did the rock test and there had to be like 30* of crank rotation before the rotor would respond.  Pulled the dizzy and the plastic was all gone pretty similar to the condition of the body and frame so that was it for that car.

The other car was a 72 that was a friends.  He didn't drive it that much and like mine it just wasn't running right (this was before mine) so we chased a few things and I never did do the rock test.  He was driving it to my house to do further testing and it quit.  I went to get him and we messed with it a bit and it was cranking funny and shooting flames out the carb. 

We towed the car home and he wanted to shoot video of the flames.  His car and at the time I had spare engines so if there was a serious problem or it became a more serious problem it was an easy fix.  This wasn't a show car.  It cranked over like one turn shot a big flame out the carb then it sucked the flame back in and started and was running reasonably well. We were able to drive it into the garage.  Once it was in he revved it and luckily had the camera rolling because he got more flames as it died.  We slapped a $30 parts store branded chain in that thing and it was back on the road later that day.

The 73 that I have had for 30 years now also lost its plastic around that same time period.  It wasn't the cars original engine so no idea how many miles were on it.  I had put 50K on it since I had owned it and inspected it.  I was already well into the process of building another engine when it started to show signs it was getting worse and I verified that was the issue with the rock rotor test.       

[V63]

When water pumps are of need that's definitely a good time to access your continuing luck with the composite cam gear.

Remember on eldorado (FWD) it's an added bugger since the oil pan cannot be readily  lowered to facilitate fitting the bottom of the timing cover and seal.

["Cadillac Kid" Greg Surfas 15364]

Just to add a bit of confusion, here is the timing set of a 1975 -500 with over 200K "on the clock". I pulled the motor apart for the block and crank.
Greg Surfas

[The Tassie Devil(le)]

G'day Greg,

That gear is on the very edge of losing its teeth covering.

Bruce.

["Cadillac Kid" Greg Surfas 15364]

Yes Bruce "on the verge" with WELL OVER 200,000 miles on the motor. The motor ran well and was pulled from a car that went to the crusher because of it's (the body) condition. Just posting this to confirm the variation in age/mileage conditions of the timing set. Aside from a bit of congealed oil there was no Nylon debris in the oil pan and here is a shot of the front of the timing cover. Looks almost new
Greg Surfas

[J. Jackson]

Here's the results of todays work.  Performed compression test cold engine, carb open, dry cylinders.  145 lowest and 153 highest.  I could not turn the engine by hand with the plugs out,  Didnt have a 9/16 x18 bolt for the crankshaft.  Timing light looked very steady with # 1 cylinder on the light.  I put in a new set of plugs and no change.  I'm beginning to look at other causes of my miss unless you guys think I should go beyond the timing light test for the timing chain/gear.  I did put a lot of pressure on the PS pump belt trying to turn the engine and I thought the dang thing was going to break.

[bcroe]

Just to add a bit of confusion, here is the timing set of a 1975 -500 with over 200K "on the clock". I pulled the motor apart for the block and crank.  Greg Surfas.

That plastic is cracked all over, getting ready to
jam up the oil pickup.  Aside from that, the wear
and chain slack do affect engine performance, for
those of us who expect near new performance.  Had
a few cases reported.  Bruce Roe