2008 6.2 No Compression in two cylinders

[mbfireboat]

My 2008 Yukon Denali with 105K miles suddenly started running really bad this week. The traction control/stabilitrac light came on at the same time.

I took it to a local shop which said I need to take it to a dealer. They felt possibly it was a bad fuel pump module (which has physical damage) and a new one would have to be programmed by the dealer.

I drove it a few miles to Williamson Cadillac here in Miami. The service advisor called me the next day and said that diagnostics points to no compression in cylinders in 2 and 4. He said that he has seen this several times before with the 6.2 where it is a broken valve spring. However, it will be a week before they can fully check out the engine.

I have always cared for my Yukon with regular maintenance and do not drive it hard.

Obviously the engine is out of warranty but I was wondering how big of an issue is the broken valve and/or spring with the non-AFM 6.2 engine?

I haven't seen much on the internet as everything seems to be related to the AFM engines.

I am hoping it is just a simple valve spring replacement but I am aware that if a valve dropped and hit a piston, I am looking at a new engine. I'll keep you guys posted as I find out more. Thanks.

 

[Crf450r420]

 

First DOHC car was a 1912 Peugeot, and Duesenberg DOHC did not appear until 1928. You have the logic reversed though in regards to rpm requirements. DOHC's main advantage is that they can rev higher than a cam in block design. This is simply due to simple layout, with far fewer parts having to start and stop while engine is running. All that weight in the lifter, push rod, rocker arm, and valve train all add together and will limit engine speed because they cannot stop the parts fast enough for them to change direction. OHC typically has cam on follower. Only two parts have to change direction when engine is running, assuming they are using a hydraulic follower. A main advantage to overhead cam is you can control valve action a lot easier, and it reduces frictional losses. I have yet to hear a Ford guy start a conversation by saying push rod engine is outdated. Need only look at NASCAR to see a cam in block 358 cu inch engine put out over 800 horse power to see that push rod engines are not dead yet. They also run those engines over 9000 rpm for 500 miles, and it is still running at the end, and will likely be run a few more times.

 

This sort of a chicken and egg story about what came first. The reduced number of parts, and weight reduction allowed manufacturers to rev engines higher, and that gives you a higher HP number(for some reason consumers are brain washed into thinking HP is real). Early roller cams used a rev kit, that added another spring to ensure that the roller lifter would stay in contact with the camshaft. Without that rev kit, it was not hard to get the pistons to aid in closing the valves due to roller lifter floating off of the camshaft lobe instead of changing direction to allow the valve to close. They rev the engine higher with DOHC because they can, not because they have to.

 

Those engines that were wiping out cam lobes on number 5 cylinder (usually it was number 5 cylinder) were usually the 305 Chev engine. There were a couple of other GM engines that were prone to this as well.

 

You are confused when it comes to failure rate numbers. Just because GM builds more engines than you does not mean they have a lower failure rate. If CompCams has a failure rate of one in a thousand, it is always one in a thousand, regardless of number of engines built. You can build a thousand and have one failure, GM can make 10 thousand and will have 10 times the number of failures you had. The rate is the same. That is why they use rates in statistics. It makes it all relative.

 

Intake velocity is what helps build torque. And that has little to do with "valve velocity". Intake velocity is important due to the ability to get more air into the cylinder when it is at a high velocity. The faster the engine runs, the less time there is to fill the cylinder with air. Long duration, and high lift helps overcome the time open issue. This is all basic engine 101 stuff. BTW, the reason for the double spring is simply due inherant spring resonance at about 6k rpm. The internal second spring on stock engines is usually a damper spring that will help control the outer spring from resonating.

If I was wrong by the first on by a few years, not a big deal, but I think I got it right.

 

http://en.wikipedia.org/wiki/Duesenberg_Straight-8_engine

 

If you have never had it happen doesn't mean it doesn't exist. I was stating my instances, so your statement would only be about yours and doesn't change mine. I've had it come up. As for the revs, if the motor doesn't build good low end torque you will find you rev it more when you drive it as that is they only way to get it to move. Does OHC allow for more revs, yes. Does OHV allow for more low end torque, yes.

 

I said that if the percent is the same then you have more complaints on the internet because there are more failures. The failure rate doesn't come into play with news reports and issue posts on the internet. The more built the more issues posts you will see and the failure rate will be overlooked as it is posted up in this way with no overview. Basic math 101 buddy. 1% of 100 is way lower than 1 of 10,000. I'll stand by my statement. You want to correct my by changing what I said and that doesn't work.

 

As for aggressive ramp rates not building torque with valve velocity and engine 101, that is several courses passed the 101 intros. If they didn't build torque there would be no reason to push the ramp rate up and you could just run a crappy old flat tappet lobe with your roller lifters. However you will be missing out on power in your motors, especially in lower rpms. You can build yours as you like.

 

As for exhaust lobes wiping out, it happened on Fords also. Like the F150 trucks in the 80's. Not really sure why you were getting so exact about the Chevy motors and don't really care.

 

Beehive springs standalone with almost no resonance and that is the great part of them, but the do require a better build to hold up. So, if you want reliability and don't need to rev to the moon with hydraulic lifter then doubles are a better choice over beehives. They also cost a lot less for a reliable set.

[Crf450r420]

 

80k? LOL!! Where the hell did you pull that number from?? Maybe in the early 80's just prior to FI - those carbs were bandaids for a problem that didn't exist .. but that's another topic for another day. Even recently I've had Edelbrock carburetors 12 - 20 years old that needed NOTHING to run right. I've run Rochester Quadrajets (and they had the nickname QuadraJUNK back in the day too ... go figure) that were OVER 40 years old with who knows how many miles on them, than ran fine ... aside from vapor lock when hot. Just hold pedal to the floor on a hot summer day, and it would start up within 10 seconds. With maintenance those old V8's will last every bit as long as a modern engine, and probably longer. Those cars/trucks didn't burn oil and tick right off the showroom floor either ...

 

Doug is right about the 305 too - they closer you get to today, the more you start to see failure prone components. It's been a gradual decline. The 305 was the economy engine - the 350 rarely saw issues like that, that weren't due to lack of maintenance.

 

FYI my family has personally run carbed vehicles WELL OVER 150k miles WITHOUT a SINGLE failure back in the day. That's towing tag-along campers too. NEVER used or leaked a drop of oil either. Oil changes, points/condenser adjustment/replacement when needed, cap/rotor replacement, spark plugs, wires, and air filters. All those vehicles were sold and continued on long after we were finished with them. It wasn't until my ol' man traded his '76 Torino for a '83 Escort did the problems start. That car had 3 COMPLETE carburetors on it before it hit 40k miles. By the mid 70's, it was common knowledge that the smaller cars were typically unreliable, and the bigger luxury cars were more reliable - that's just how it was. By the late 70's into the 80's, government meddling had turned that into a hit or miss affair.

 

I know a guy that got well over 300k miles on the ORIGINAL Chrysler slant 6 in Dodge Dart - carb & points. Thing had hit everything but the lottery, but was still running and driving when it was finally retired by a telephone pole that jumped out in front of it ...

 

Government meddling has caused just as many reliability issues as greedy manufacturing practices. Remember - the entire WORLD was getting by with carburetors and POINTS ignition for 75+ years before solid state ignitions, and 85+ years before fuel injection. Just because the only carbureted cars you've ever seen are now 30 -40 + years old, neglected, and worn out, doesn't mean that's how they were when they were new. American cars USED to be the envy of the world. We built the BEST. Thanks to the government, the rest of the world has had plenty of opportunity to play catch-up - decades, in fact.

 

Prior to '73 cars were all built like tanks - that's why most of them are still here 42 years later - take a browse on eBay for pre-'73 vehicles. People are too spoiled now to deal with adjusting points or even waiting for a car to warm up. They'd melt down! There was a time for DECADES where this was just the way it was. Even in those days there were plenty of guys that didn't get it. Our family never had a problem. You checked them at certain intervals, and replaced them when needed - it took a half hour tops to do a dual point Ford with the distributor right out front, AND it didn't cost an arm and a leg.

 

Don't get me wrong - I'm not 100% against FI and computers -- I AM 100% against UNRELIABLE FI & computers. So far in that respect I can't complain about my truck .... but it's only got 75k miles on it, so I should hope the hell not!

 

Ford got it right in '86 with their sequential fuel injection in the Crown Vic/Grand Marquis, trucks, and the Mustang. It's well known that those injectors AND fuel pumps will run WELL NORTH of 300k miles. Lets see any of our vehicles today pull that one off ....

 

It can be done, if they would just build them correctly ....

Your right my number is probably off it was more like 60k and you were rebuilding the carb. I'm not saying it was trash as that point, but it needed cleaning and little service parts plus gaskets. Do some last much longer sure. Today most will only last a few years before the ethanol will retire them.

 

You can assume I'm some young kid that has only seen a carb car when it was already 30 years old, but you look like an ass. Where you the one at the start of this thread telling me your going to sell your truck and there is no way spring should fail at 100k and now your on board with the concept and idea, wtf?

 

Back to the guy who needed help. Second opinion. Have them show you the broken parts. Do some tests. You can youtube how to do most of this stuff with little to no money. good luck.

[bobejones]

Loss of power coupled with traction light??? Something smells off in the loss of compression diagnosis. ABS sensors use a tach feed back to the PCM to determine wheel slip and retard timing until slip condition resolves.

 

If the you are down on 2 cylinders, the engine will run VERY rough. Not just loss of power, but ROUGH!!! Like shaking because the engine is no longer balanced. You will also have abnormal sounds from the engine.

 

If your engine idles smooth, go find another mechanic and tell that dealer to refund you for wasting your time.

 

Also, you can run a leakdown test on those cylinders at TDC for those cylinders to determine where the leak is. Not rocket science.

[Jsdirt]

I don't know what failing valve spring has to do with your insistance that all carbs are junk, but whatever .... my point remains - a $50k truck should NOT be breaking valve springs at 100k miles PERIOD.

 

I guess I'm the only guy on the planet that got 5 years out of a Quadrajet, and 12 years out of an Edelbrock in my El Camino, and close to 20 years out of a Edelbrock on a '81 c10 without touching ONE gasket on any of them. MA dumps more ethanol in the fuel than anyplace in the country (have seen my Flex read 19.7% on some fillups). Current Edelbrock in our '89 S10 has been in there since '11. Driven daily, except it sat all of '13 - no leaks, no problems. Started right up on -6° February morning, even when I forgot to plug in the block heater. Guess I'm a lucky guy!

 

Didn't mean to derail this topic with all this B/S. Some people just don't get it. That's fine - more carburetors for me!

 

 

How goes the battle, Fireboat??? Don't leave us hanging - the thread will be 24 pages if you do ....

[Doug_Scott]

 

I said that if the percent is the same then you have more complaints on the internet because there are more failures. The failure rate doesn't come into play with news reports and issue posts on the internet. The more built the more issues posts you will see and the failure rate will be overlooked as it is posted up in this way with no overview. Basic math 101 buddy. 1% of 100 is way lower than 1 of 10,000. I'll stand by my statement. You want to correct my by changing what I said and that doesn't work.

 

I am going to limit my reply to this one point, can't be bothered to continue trying to explain something that you just do not want to see.. If you think that 1% of 100 is way lower than 1 of 10,000 then you are simply wrong. 1% of 100 is 1. 1 of 10,000 is 0.01%. 1% is 100 times larger than 0.01%. Note that I am not changing what you said. That is word for word what you wrote.

 

Keep in mind that I have no idea what you are trying to convey, so I have to read what is there, then try to make some sense of it. Only you know what you were trying to say. Not my fault if you are not being clear enough in what you write.

[vettelovralexand]

If it really is a broken valve spring, it is a super easy fix if you are willing to do it yourself and buy a spring compressing tool. The hardest part is getting setup so you dont drop a valve in a cylinder. They have tools, but some people push rope into the cylinder through the spark plug hole and then bring the cylinder toward tdc to hold the valve up while the spring is swapped.

[Doug_Scott]

Extremeley easy to change a valve spring, without using rope. Take your compression tester adaptor, screw into plug hole, loosen both rockers so valves are not being held open, and connect air line to adaptor. Quality compression testers have a standard air line fitting on the adaptor.

 

The only time I would expect to see a stock engine have a valve spring fail by breaking is on first run in. A defect that causes a break will be in manufacturing, and will show up right away. Stock valve springs are one of those parts that is not where there is any money to save. Cost per failure is far too high.

[Jsdirt]

Yeah ... but your talkin' mega-lift with a camshaft that wouldn't even fit in our engines. My engine is 100% stock. I've seen engines with 300 -400k+ miles with all original internals - granted, not one was built in the 21st century ...

 

No excuse for breaking a valve spring on a stock engine, let alone at 100k miles! I've seen them wear out and get weak .. but never break. There's cars out there 100+ years old with original valve springs, still running!

 

This is just a blatant example of a company trying to save a buck by nickel-and-diming things they shouldn't.

[Doug_Scott]

Loss of power coupled with traction light??? Something smells off in the loss of compression diagnosis. ABS sensors use a tach feed back to the PCM to determine wheel slip and retard timing until slip condition resolves.

 

 

When did they stop using the wheel sensors to detect wheel speed difference (as in spinning)? Can't see how monitoriing the tach feed will ever show wheel slippage when the vehicle has a torque converter that is not locked up.

[Crf450r420]

Right because you get parts from Germany to save money.

[Jsdirt]

Doesn't matter where they came from - they shouldn't be breaking on a STOCK engine.

[Crf450r420]

Obvious typo

[Crf450r420]

Your right. Sell your truck quick.

[Jsdirt]

Thanks for the tip pal. Good to see another sucker on here with his head up GM's ass. Throw another $60 grand at 'em - they love ya!

 

Proof positive - there really is an ass for every seat ....

[Doug_Scott]

The Edelbrock carbs are the old Carter AFB/AVS carbs. Back in my street racing days the only carbs I would use were the Carter AFB Competition Series. In my 78 King Cobra I put an older factory Ford 2x4bbl intake manifold with 2 Carter AFB CS 500CFM. Back then Carter sold Strip Kits for the CS series. The kit contained just about the complete selection of jets and metering rods in one box. The kit ran just under $20. I bought two kits, one for each carb. Over the next 5 or 6 years that car went through many engine combinations. Other than the original engine, every engine was dual 4bbl. The wildest version of the car was a BOSS 351, stock short block(other than balancing), hydraulic cam with Rhoads lifters, Weiand tunnel ram w/2 AFB, 625cfm, 3500 stall convertor, manual shift valve body C4 trans, rear frame with tubs, 14 point cage, 9 inch Ford differential with 35 spline spool, Summers axles, and 5.38 gears. Ran on Weld Wheels, 15x13 and 15x3.5. Cannot recall what tires I ran, but they were not marked as being 'not for highway use' Drove this car every day(other than rain days) from May 1 to Oct 30. Only had the car in this configuration for the last 18 months I owned it(actually, it owned me). The strip kits allowed you to change metering rods in under a minute with only a small phillips screw driver. Changing jets took about 5 minutes for both carbs. Those carbs were cheap to buy compared to Holley carbs. Jetting was a lot faster than the Holley carb.

Car was just barely street legal, the only big issue with it was the spool in the rear end. Lane changes had to be planned ahead of time, Going around corners at intersections had to be done at fairly skow speed, going to faxt, the car would not turn, it would just push the volkswagon beetle front tires in a straight line. Getting caught in the rain meant parking the car where you were, due to not being able to turn the car on wet pavement. Best time I ever got with it was 11.2 in the quarter with full exhaust connected.and street tires.