Pushrods too short = misfire?
Thread Starter
Joined: Nov 2001
Posts: 10,060
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From: Canton, GA
Pushrods too short = misfire? Got my car running but have a misfire causing real headaches. I bought 7.350" pushrods because I thought I was going to run milled 5.7L heads. Well it turns out I got 5.3L heads instaed and they are not milled. Could too short of pushrods cause misfires on a few cylinders? Seems like it would affect all of them if anything?
I am nearing the end of things to check. Checked for continuity on the injector wiring and coil wiring and all was fine. Still not convinced that my home-done plug wires aren't the problem. The misfires seem to be moving though? Freaking weird? Last night only #4, today #4 and #5. Sheesh, what a PITA
I am nearing the end of things to check. Checked for continuity on the injector wiring and coil wiring and all was fine. Still not convinced that my home-done plug wires aren't the problem. The misfires seem to be moving though? Freaking weird? Last night only #4, today #4 and #5. Sheesh, what a PITA
Re: Pushrods too short = misfire? Remember, it's going to depends on more than just the head being milled or not -- I have run 7.350's on several combinations. It's all about geometry -- making sure the rocker sweeps the valve stem properly, and keeping lifter preload in the sweet spot. All kinds of things can affect optimum pushrod length: big cams with smaller base circle, different lifters with different preload requirements, taller valves/springs with shimmed up rockers, skimmed heads, etc.
BUT ... for simplicity, since its so easy for you to pull a valve cover (I'm jealous, BTW) see how much lifter preload you have. If it's next to nothing, you probably found your knock
Also, when watching misfires on a scanner you'll often see the effect of one cylinder misfiring affect the next hole in the firing order. So, if #5 is dead, you'll see misfire counts on #4 too since its next in the firing order. The PCM sees the crank hasn't fully accelerated back to what it was on all the other cylinders, so it adds counts to #4 as well -- just not as many as #5. Make sense?
BUT ... for simplicity, since its so easy for you to pull a valve cover (I'm jealous, BTW) see how much lifter preload you have. If it's next to nothing, you probably found your knock
Also, when watching misfires on a scanner you'll often see the effect of one cylinder misfiring affect the next hole in the firing order. So, if #5 is dead, you'll see misfire counts on #4 too since its next in the firing order. The PCM sees the crank hasn't fully accelerated back to what it was on all the other cylinders, so it adds counts to #4 as well -- just not as many as #5. Make sense?
Re: Pushrods too short = misfire? How much of a turn did it take to get 22 ftlb on the rocker. Should be between a 1/4 and 1/2 turn for proper preload. You need to check all of them as they will differ from one to the next. Is the cam a 30 under base circle? Check the plugs to see how those cyclinders are running.
Thread Starter
Joined: Nov 2001
Posts: 10,060
Likes: 33
From: Canton, GA
Re: Pushrods too short = misfire? Thanks Brains....I think I found the problem with #5. Seems a few of the connectors on the harness were bent down and making poor contact. I had an extra harness and spliced a new connector for #5 and it is running smoother now.
I installed some stock pushrods just out of curiosity. I don't know all those gorey details about the cam. Just know what is on the card. It is a Comp XE-R grind 232/234 .595/.598 112.
So if i tighten the rockers to the point where the pushrod won't turn, then add 1/4 - 1/2 turn, I should be at 22ftlbs? I will try this with the 7.350" pushrods and see what happens. If it is excessive, I guess I need 7.4"s.
I installed some stock pushrods just out of curiosity. I don't know all those gorey details about the cam. Just know what is on the card. It is a Comp XE-R grind 232/234 .595/.598 112.
So if i tighten the rockers to the point where the pushrod won't turn, then add 1/4 - 1/2 turn, I should be at 22ftlbs? I will try this with the 7.350" pushrods and see what happens. If it is excessive, I guess I need 7.4"s.
Re: Pushrods too short = misfire? I installed some stock pushrods just out of curiosity. I don't know all those gorey details about the cam. Just know what is on the card. It is a Comp XE-R grind 232/234 .595/.598 112.
So if i tighten the rockers to the point where the pushrod won't turn, then add 1/4 - 1/2 turn, I should be at 22ftlbs? I will try this with the 7.350" pushrods and see what happens. If it is excessive, I guess I need 7.4"s.
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TECH Fanatic
Joined: Nov 2001
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From: SW Florida
Re: Pushrods too short = misfire? Todd ...
with your cam and unmilled heads, you need the stock 7.40" rods.
As far as checking preload, just look at this:
__________________________________________________ _______
CHECKING LIFTER PRELOAD
Turning the engine, bring one of the cylinders to TDC. The valves will be fully closed.
Now, fully loosen one of the rocker arm bolts on that cyl.
Then start turning the rocker arm bolt down until the lifter is at "zero lash". You will be able to tell it is at zero lash when the rocker bolt gets very hard to turn. It gets hard to turn because the valve spring starts to compress. Again, stop turning the bolt as soon as you feel resistance to turning.
Mark the bolt with a marker.
Now count how many turns it takes to completely tighten the rocker bolt to 22 FT/LB.
The bolt should have turned between 1 3/4 to 2 turns for the acceptable GM lifter peload.
If you don't get the 1 3/4 to 2 bolt turns of preload, you need a different length pushrod. (IE: you need a shorter rod if it takes much more than 2 turns, and a longer rod if it takes much less than 1 3/4 turns).
I really think anywhere between 1 to 2 turns is acceptable. Remember, hyd. lifters can operate within a fairly large margin of preload as long as it's not excessive.
NOTE: You really only need to do this on one cylinder. The rest will be the same.
NOTE: If you have aftermarket Comp Cams “Type R” lifters, I understand that those like less preload than stock lifters. Instead of 1 3/4 to 2 full turns as indicated above, about 1/2 to 1 turn may be more acceptable.
NOTE: Allways run the engine prior to checking the preload so as to get the lifters pumped up. This is important if you just installed new lifters or had the old ones out while installing a new cam.
Ron,
with your cam and unmilled heads, you need the stock 7.40" rods.
As far as checking preload, just look at this:
__________________________________________________ _______
CHECKING LIFTER PRELOAD
Turning the engine, bring one of the cylinders to TDC. The valves will be fully closed.
Now, fully loosen one of the rocker arm bolts on that cyl.
Then start turning the rocker arm bolt down until the lifter is at "zero lash". You will be able to tell it is at zero lash when the rocker bolt gets very hard to turn. It gets hard to turn because the valve spring starts to compress. Again, stop turning the bolt as soon as you feel resistance to turning.
Mark the bolt with a marker.
Now count how many turns it takes to completely tighten the rocker bolt to 22 FT/LB.
The bolt should have turned between 1 3/4 to 2 turns for the acceptable GM lifter peload.
If you don't get the 1 3/4 to 2 bolt turns of preload, you need a different length pushrod. (IE: you need a shorter rod if it takes much more than 2 turns, and a longer rod if it takes much less than 1 3/4 turns).
I really think anywhere between 1 to 2 turns is acceptable. Remember, hyd. lifters can operate within a fairly large margin of preload as long as it's not excessive.
NOTE: You really only need to do this on one cylinder. The rest will be the same.
NOTE: If you have aftermarket Comp Cams “Type R” lifters, I understand that those like less preload than stock lifters. Instead of 1 3/4 to 2 full turns as indicated above, about 1/2 to 1 turn may be more acceptable.
NOTE: Allways run the engine prior to checking the preload so as to get the lifters pumped up. This is important if you just installed new lifters or had the old ones out while installing a new cam.
Ron,
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Verdad Gallardo TECH Fanatic
Joined: Nov 2001
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From: SW Florida
Re: Pushrods too short = misfire? very true...
BTW, Your cam has about a .030" smaller cam centerline to lobe heel distance than your stock cam. That would mean that you really would need .030" longer pushrods than stock, however, the hardened 7.40" rods are actually .015" longer than the stock rods already. (the stock rods are actually 7.385") The 7.40" hardened rods are actually 7.40" long.
So, use these rods and you will only has .015" less preload than a bone stock engine. In actuality, having only .015" less preload will allow the engine to spin a little faster before the lifters start to pump up.
You will still have lots of preload, so don't worry.
Ron,
BTW, Your cam has about a .030" smaller cam centerline to lobe heel distance than your stock cam. That would mean that you really would need .030" longer pushrods than stock, however, the hardened 7.40" rods are actually .015" longer than the stock rods already. (the stock rods are actually 7.385") The 7.40" hardened rods are actually 7.40" long.
So, use these rods and you will only has .015" less preload than a bone stock engine. In actuality, having only .015" less preload will allow the engine to spin a little faster before the lifters start to pump up.
You will still have lots of preload, so don't worry.
Ron,
Re: Pushrods too short = misfire? You must check every single rocker. There will be some variance from one to another. Correct preload for after market lifters is more like 1/4 to 1/2 turn. Just because one is set correctly doesn't mean you just torque the rest down. I have anywhere from no shim to .030 shim on my rocker to get the right preload. No noises lifters on my car. Take the time to do it right the first time.
TECH Fanatic
Joined: Nov 2001
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From: SW Florida
Re: Pushrods too short = misfire? QUOTE: "Correct preload for after market lifters is more like 1/4 to 1/2 turn".
__________________________________________________ _______
How do you know he has aftermarket lifters?
As for stock lifters; It's 1 3/4 to 2 turns from zero preload until the bolts are fully tightened (we have seen this on 2 separate bone stock engines.
He can check everyone of them if he wants, but I don't think he will find much variation between any of them. All these cam lobes are all ground the same from cyl to cyl, and the pushrods all measure out to within about .002" if you mic each one of them.
__________________________________________________ _______
How do you know he has aftermarket lifters?
As for stock lifters; It's 1 3/4 to 2 turns from zero preload until the bolts are fully tightened (we have seen this on 2 separate bone stock engines.
He can check everyone of them if he wants, but I don't think he will find much variation between any of them. All these cam lobes are all ground the same from cyl to cyl, and the pushrods all measure out to within about .002" if you mic each one of them.
Thread Starter
Joined: Nov 2001
Posts: 10,060
Likes: 33
From: Canton, GA
Re: Pushrods too short = misfire? Stock lifters because I didn't want to fuss with the sensitive Comp Rs. Heard they are very sensitive to correct preload. I will have to kepp the car sitting until I find some pushrods. Thanks again! 
TECH Fanatic
Joined: Nov 2001
Posts: 1,975
Likes: 2
From: SW Florida
Re: Pushrods too short = misfire? Todd ... Just get a set of hardened 7.40" and your preload will be well within limits with the cam and heads you have.
Good luck,
Ron (30 days from RETIREMENT)
P.S. Your car is realllly lookin good!
Good luck,
Ron (30 days from RETIREMENT)
P.S. Your car is realllly lookin good!
Re: Pushrods too short = misfire? [quote]
QUOTE: "Correct preload for after market lifters is more like 1/4 to 1/2 turn".
__________________________________________________ _______
Is this correct with the Comp and Crane stock replacement lifters? Or are you talking about the Comp R's only, with that statement??
Dan
QUOTE: "Correct preload for after market lifters is more like 1/4 to 1/2 turn".
__________________________________________________ _______
Is this correct with the Comp and Crane stock replacement lifters? Or are you talking about the Comp R's only, with that statement??
Dan
Re: Pushrods too short = misfire? Is this correct with the Comp and Crane stock replacement lifters? Or are you talking about the Comp R's only, with that statement??
We do the same preload on both comp r and crane lifters. Both are quiet when set up correctly.
We do the same preload on both comp r and crane lifters. Both are quiet when set up correctly.
Re: Pushrods too short = misfire?
Is this correct with the Comp and Crane stock replacement lifters? Or are you talking about the Comp R's only, with that statement??
We do the same preload on both comp r and crane lifters. Both are quiet when set up correctly.
We do the same preload on both comp r and crane lifters. Both are quiet when set up correctly.
Thanks! I was wondering cause I'm running .010-.020" of preload on my Comp OEM lifters and didn't know if that was to little or not. I was told from others to use .035-.050"...
Dan
Re: Pushrods too short = misfire?
todd, did you do a crank re-learn? since you have moved the crank sensor and crank from its original positions, it could also cause misfires.
Dan