My 1981 250 started backfiring today. It had weak power going up a hill. Someone told me - when they heard the backfire - that I had a worn camshaft.
I looked online and the symptoms match. Plus I had put Ethanol in it the other day because I was at that gas pump. One article said Ethanol or water in the gas may cause that too.
My question is... Could I get away with just replacing the Lifters and Pushrods until I can afford a rebuild?
If it's started backfiring from a cam issue, you've wiped a lobe and there isn't anything you can do but change the cam.
You can pull the valve cover and watch the rockers. One will probably not be moving much if it's a cam issue. Also it will probably cough on every cycle.
That said, backfiring can also be caused by timing and mixture issues. I'd check the timing first.
Thanks for the quick reply. I will check the rockers. That gasket is only a few bucks. This was an old farm truck I got for $400
If you have the original 1981 carb, running Ethanol through it may have some unpleasant aftereffects on your carb. I made a cross-country trip in 1980 from Los Angeles to Iowa and back to L.A. with a buddy of mine. Along the way we ran Gasahol (remember that?) going through the midwest states that were selling it. By the time we got back to California, the car was barely running, backfiring, coughing, and so on. Turns out that the Gasahol had pretty much eaten the carb insides. My buddy had to replace the carb. There was no way to fix it. So you might take a look at the carb as well as the cam.
Check your timing first. Sometimes the cam can slide back out of the timing gear and cause similar problems. If you suspect this, when you pull the distributor out, the gear on the cam should be centered in the hole, if its not, then it has slid back.
How long since the last tune up?
replacing a lifter or push rod will not help a bad cam lobe.
Running ethanol can definatly cause lean misfire. Gaskets are cheap, but if you don't find a bad lobe, put regular back in and see if the problem disappears.
Thanks for the replies. The Last Tune Up was in May. New Plugs, Wires and new internals in the HEI.
I did not run Ethanol on purpose. I stopped at a Shell Station because I needed gas right then.
Many stations are using Ethanol now. I bet they are getting a Tax Refund for it.
I usually run 92 Octane in all my vehicles. Will let you know in a few days how the engine is doing.
You would hear a lifter making noisy if the cam was very bad. A lean condition will cause backfires if its lean enough, and ethanal could very well be the problem. Add a bunch of 92 octane and see what happens. Your engine should have about 8 to 1 compression so the standard 87 octane will be plenty for it.
Joe
If I have a cam that is "bad" what would I see when I had it on the bench?
Also, what is the abosolutely best oil and weight and or additives to use to keep the cam from going bad again?
Thanks.
Ken
Some cams can have chrome plating that rubs off or if the metal just wears down over time. Here is a link with pictures of bad cams but it is for AIRPLANES.
http://www.sacskyranch.com/camshaft.htmThe theory is the same plus there is pictures on this page.
I decline to go into oil weight and additives. I will just say change the oil every few months and keep it to the proper level.
Dear Joe H. I cant hear the lifters because I have an exhaust leak. I will take your advice and put good gas back in it. Thanks Bud.
Where is it leaking from? If its the head flange, then the intake is probably leaking to which will cause a lean backfire.
You will need a set of calipers or a micrometer to measure your cam. Check the lift on each lobe, the lowest one is the worn one. Joe
Exhaust leak is where the manifold meets the pipe. It has two exhausts on the manifold. Some have donut gaskets there. I have a big gap but the bolts are rusted.
There may be a leak on the farthest back flange because there is a gooey substance there.
If I take the cam out I hope to be able to replace it since I would have it down that far. I would like to "Mike it" just to see the evidence.
Pine Bluff Dude,
I had a very similar problem.
My car, which used to start at first crank, started to take a lot of cranking to fire. It then wanted to stall, I had to keep the revs up until it started to warm up. When warm it ran fine until I needed more power (enter freeway or up hills) then it coughed and backfired through the carb. When I got to speed it ran smoothly.
The answer was a failed cam. I'm rebuilding the engine as you read this.
I hope that is not your problem but if you have those symptoms then it may be.
Bill
Thanks for the specifics on how it backfired and operated. That will be helpful in my diagnosis.
I have not drove the truck for a week because I am a wus. I have just used my LTD Station Wagon as a work truck instead.
Today my local mechanic mentor - told me to take off the Valve Cover and watch the rockers.
Found a picture of a Worn Cam from a 292. I know this sounds like a weird link but I checked it and got the photo
http://www.employees.org/~devo/65chevy/292rebuild/100_0199.jpgIf you want to see the whole rebuild this page has photos. The pics are big so it is slow to load on Dial-up. The photos look like 200 x 300 size but if you click Properties and copy the picture address... you can open it in another Tab and it is 1280 x 800 pictures. Here is the link
http://www.employees.org/~devo/65chevy/new.motor.html
Good News. I pulled the Valve Cover and found a Rocker Arm sitting sideways. The pushrod was only rubbing the side of the rocker and not pushing on the bottom.
As far as the Worn Cam... I have to wait till I have the money and gaskets before I take it down that far.
So for now I am using the Monster Under the Bed Theory. If I dont look under the bed... I wont see any monsters.
You may not be out of the woods yet. Was the pushrod going up and down the same amount as the others? You could still have a worn cam or collapsed lifter. If you don't want to start the engine with the valve cover off, you can pull the plugs and crank it over with the starter while watching things.
The only way the pushrod can come out of the rocker is:
1- Adjustment was way, WAY too loose
2- Cam wiped resulting in the lifter/pushrod dropping enough to come out of the rocker
3- Lifter collapsed resulting in the pushrod dropping enough to come out of the rocker
Unless the lifter is moving the same amount AND at the same distance from the top of the head, you'll need to pull the side covers and look at the lifter. A visual inspection will tell if it's collapsed. They are cheap.
I guess it's possible the adjustment nut backed off enough to let the pushrod out but I've never heard of that happening....not to say it can't.
Gentlemen,
Can you explain the benefits and RPM range of running an RV cam? How do you specify an RV cam when you order one for a Chevy 250?
Thanks.
Ken
The nut on the rocker did back off. The Thread Protrusion... was only one or two threads sticking out of the nut. The others had about 2 or 3 threads sticking out.
I have not put the valve cover back on yet. I had the spark plugs out so I could turn it over by pulling on the belts. So I will check those other two items you mentioned.
Is there a specific way to look at a Lifter and know if it is collapsed?
Benefits and RPM range of running an RV cam? The word RV cam is a generic term. It just means NOT STOCK but NOT Hot Rod. They are designed for breathing better at Low RPM. If you are pulling a heavy load while towing or driving a heavy RV.
Just look at the different cams available and BE HONEST with the RPM you are running. You are not at a Drag Strip so in town or on the Highway you will be running less than 4000 RPM. Just look at the cams and pick the part number. If you want... print the page and take it to the car parts store with you and tell the guy what you intend to use it for.
You can tell by looking at it. It's obvious compared to a good one.
El Viejo,
The main benefit of an RV cam is that it gives you more torque (and a little more horsepower, too) than a stock cam, and generally in the same RPM range as the stocker. They're designed to work best in an rpm range of 1000 - 5000 rpm. Most of them have between .480" and .500" lift, and advertised duration between 250-260 degrees. They give you a normal idle, and pull very stongly from just above idle to the upper rpm limit. They also will give you good fuel mileage if you aren't TOO much of a leadfoot, and they have a strong vacuum signal. All in all, they're a great street cam. Used with headers and an aftermarket intake manifold, they put out an impressive amount of torque when you put the hammer down. They're also great if you're towing a boat or trailer. LOTS of low and midrange grunt. Most of the aftermarket cam grinders will specify that a particular cam is an RV cam in their catalog or web site, but if they don't, you can tell from the duration and lift specs.
Gentlemen,
Thank you all.
Ken