Replaced header gasket now loss of power

[clayton]

I need some help. Recently replaced a header gasket, I had a small leak where header bolted to cylinder head. Fixed the leak no problem. But now the car runs with low vacuum ( bounces between 5 and 10). Also it's running a lot leaner than normal. My car always ran kinda rich but now runs between lean and stoich area on my air and fuel gauge. Also at idle it surges on and off from 800 to 1200 rom. End result is now loss of my low end power. The car RIPPED off the line before, you couldn't keep the tires from shredding asphalt. Now they just chirp and slowly climbs in rpm until you get to around 4k then the engine will come alive. Same with down shifting for example ; cruising in 5th gear around 3k on the tach, throwing it down in 4th and smashing on the gas it takes off like a boat anchor. I can't find any vacuum leaks and literally all I did was replace a header gasket. Please help...I want my 331 to feel like the snappy little stroker beast she once was.

 

[cobrajeff96]

Sounds like air is slipping in and affecting the O2 sensor(s).

 

[clayton]

Sounds like air is slipping in and affecting the O2 sensor(s).

 

[clayton]

Yea man I don't know how that would be, the header gasket seems to have sealed up real nice and all the bolts are tight. Where the exhaust pipe meets the header is also bolted up tight, you can't feel any exhaust leak pushing out at all ( I just put my hand up to the header collector area while it's running).

 

[ttocs]

maybe ya didn't get a plug wire all the way on or it is shorting out now? Just close enough that sometimes when ya floor it, it makes contact and you have all 8 firing but the rest of the time it is either not firing or not completely firing. Also if you do not know, a bad plug wire can make an arking/clicking sound when they go bad

 

[cobrajeff96]

Is it header bolts or studs?

If studs, ideally the attaching nuts are distorted thread to reduce chance of loosening. If bolts, they need to be rechecked after a heat cycle.

Well, header fasteners should be rechecked whatever the situation. It could just be that.

 

[clayton]

They are header bolts. I'll re check them as soon as I get a chance. Would a slight leak cause vacuum loss and for the car to run significantly leaner though? Seems excessive to me....but these issues only occurred once I replaced the header gasket. I don't know much about vacuum on efi engines. My car runs around 10 on vacuum gauge and while accelerating it drops to 5. Then will hang around 10 while normal cruising. It is a 331 stroker 30lbs injectors, TFS heads, intake and stage 1 cam with 5 speed manual. Bbk shorty headers no cats or mufflers just straight pipe. The vacuum seems low to me...unless I'm wrong?

 

[cobrajeff96]

Could be a legit vacuum leak somewhere.

But normally if there's air seeping through a header leak it will cause the O2 to read lean and increase fuel in the mixture, not take it away.

 

[07GtS197]

Where are you getting vacuum signal from? Depending on where you fitted the vacuum gauge it could be a false reading. 5 inches of vacuum is very low. A stock setup would be around 20 inches at sea level. Even with a stage 1 cam I don’t see it affecting vacuum that much. If you didn’t already, pull the vacuum line off of the brake booster and take a reading there.

 

[clayton]

The car has a boost and vacuum gauge mounted on the dash. I can try hooking my other gauge to the line that runs to the brake booster when i get home and see what it reads. So you think i shoukd be somewhere between 15 and 20 at idle?

 

[07GtS197]

Depends on the cam specs and altitude but my guess would be closer to 15 inches of vacuum. Is your vacuum gauge hooked up to a dedicated vacuum port or tee’d? You need a dedicated vacuum source for an accurate reading.

 

[Methodical]

Did you find out the problem?

 

[clayton]

Not yet. It still is reading low vacuum. When I hook the guage to my vacuum line comming off I take to FPR it reads 14. But my gauge mounted to my dash reads 5. It still idles with crazy surges, gets horrible gas mileage and lost its low end crazy performance. I did find I have a very small and slow oil leak comming from the back of my lower intake manifold. I don't know if this is the cause of all of this or just a separate issue. I ran a smoke test on the engine and vacuum lines and have no leaks at all, no smoke comming from anywhere.

 

[95opal]

Not yet. It still is reading low vacuum. When I hook the guage to my vacuum line comming off I take to FPR it reads 14. But my gauge mounted to my dash reads 5. It still idles with crazy surges, gets horrible gas mileage and lost its low end crazy performance. I did find I have a very small and slow oil leak comming from the back of my lower intake manifold. I don't know if this is the cause of all of this or just a separate issue. I ran a smoke test on the engine and vacuum lines and have no leaks at all, no smoke comming from anywhere.

I would bet money on that lower intake leak is your problem.

 

[clayton]

I'm hoping that's it! But when I did my smoke test, shouldn't smoke have made its way out if that lower intake leak? I had no smoke anywhere...I even took it up to the local garage and they ran their smoke test and had the same result, nothing. It's quite weird to me. But yes, it has a very small slow oil leak from the back of lower manifold, and has lost a lot of its low end power. It use to rip right off the line, but now doesn't seem to pull hard until hitting close to 4k rpm. And the idle surges like it's a huge solid cam which it's not...just a tfs stage 1. Now, it hasn't been tuned yet, but it still idled somewhat decent without being tuned. So I know it's not a tuning issue

 

[Terrorist 5.0]

I'm hoping that's it! But when I did my smoke test, shouldn't smoke have made its way out if that lower intake leak? I had no smoke anywhere...I even took it up to the local garage and they ran their smoke test and had the same result, nothing. It's quite weird to me. But yes, it has a very small slow oil leak from the back of lower manifold, and has lost a lot of its low end power. It use to rip right off the line, but now doesn't seem to pull hard until hitting close to 4k rpm. And the idle surges like it's a huge solid cam which it's not...just a tfs stage 1. Now, it hasn't been tuned yet, but it still idled somewhat decent without being tuned. So I know it's not a tuning issue

Sometimes small vacuum leaks won't show up with a smoke test. I've had a vacuum leak near the firewall that was small enough that it didn't show, even with an industrial grade smoke tester. Even the smallest of vacuum leaks can freak the computer out, although mine didn't show symptoms like yours. If I were you, I would fix that lower intake leak and keep an eye out for anything that could cause a vacuum leak. You may just find the issue in the process and you are getting one issue out of the way anyway. One thing I may add is make sure the PCV boot is snug. They are known for fitting like garbage. I change my PCV components every couple oil changes since my engine is old.