Welding On Cars

A friend of mine had a 80s-ish Toyota Celica with a crack in the cast iron exhaust manifold. The crack was thin, but went from the 02
sensor to the top of the manifold. He thought it might be weldable. I thought it would just crack again as the weld would pull up the contamination from inside the manifold. We opted against trying it.
However, it made me think -- how could I go about doing this: Keep manifold on car (engine exhaust makes such a good pre-heater!), use a "cast-iron repair rod" and not kill any of the computers. As cars get more computers, the last one is a big cocnern.
Since its "practically" impossible to disconnect all the computers on a car, what can you do to protect them?
__________________ Note: To reply, replace the word 'spam' embedded in return address with 'mail'. N38.6 W121.4
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

I am a welding instructor and tried the same thing on my 85 toyota truck. It was hopeless. Just bite the bullet. Go to a junkyard and buy an exhaust manifold off another Celica of the same era. I got one for $35 for my truck. I then sand blasted it, ground off any rough edges, and painted it with high temp paint.
It was surprisingly easy to swap out the exhaust manifold. The only difficult part was getting all the manifold bolts out without stripping threads. You might want to go ahead and buy the helicoil kit for those bolts ahead of time. If you don't need it, then you can return it.
Spray some good penetrating lube on the bolts the day before so it gets time to soak in.
I replaced all of mine with new ones from Toyota.
And buy a new Oxygen sensor while you are at it.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On Sat, 04 Oct 2003 20:50:15 GMT, Ernie Leimkuhler

I believe I echoed those very same sentiments and he bought a new one..
__________________ Note: To reply, replace the word 'spam' embedded in return address with 'mail'. N38.6 W121.4
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
OTOH, I have welded exhaust manifolds with 100% success. Both my current truck and my wife's Chrysler are running exhaust manifolds I worked on.
My technique has been to get everything cleaned up, v-ed out and properly positioned, then to heat hot with a propane torch and weld with full power, lots of gas and 0.023 ER70S wire with my SP125+.
After laying down a lot of weld, I will usually pean with a flux hammer and heat red with the propane torch. Then the item is cooled slowly.
Lots of people might just say "not possilble", but I have two exhaust manifolds, a Wilton vice, an Atlas 10" lathe, an intake manifold, and a few other cast iron pieces welded up with my SP125+. Not one failure.
If I had made a bet every time somebody said "not possible", I would have a very nice TIG welder today.
wrote:

at it.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

First, realize that car electrical systems naturally produce transients as high as a couple hundred volts. So the on board computers, radios, etc have to have transient suppression built in to protect them during normal operation. Note also that the auto battery is a very good surge suppressor itself, so disconnecting the battery can be counterproductive.
Meanwhile, your arc voltage is going to be on the order of 20 volts. Any voltage transients you produce are unlikely to be greater than those encountered in normal automotive operation.
The real risk is that you'll inadvertently try to flow excessive current through circuits not designed to handle that much current. The primary way to avoid that is to make sure your welder's ground clamp is very close to the weld, and clamped directly to the material being welded, not just to some convenient place in the engine compartment (or horrors, to the bumper).
In the particular case you describe, I would unplug the oxygen sensor. It is a very low current circuit, and leaving it connected while welding on the manifold is just asking for trouble.
As to welding the manifold itself, you want to stop drill the crack, *lightly* gouge the crack with a die grinder, and *butter* on the repair weld. Don't try for full penetration. That'll pull up gunk from inside the manifold and contaminate the weld. Your bead should be proud on the surface, a patch in essence. Peen frequently as you weld to let the stresses equalize. High nickel or stainless filler is recommended.
An alternative is to torch braze the crack. You'll need to use an active flux, but this should result in a sound repair with little hazard of cracking or warping, and zero hazards to electrical systems on the car.
Gary
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
wrote:

I don't know if hot welding sparks near a battery are ideal.. Maybe replace the battery with a really large capacitor while welding on the block near the battery?

Could AC or HF start cause problems?

Butter?
Essentially a cold weld?

Probably a safer way to go. Is the surface prep the same?
__________________ Note: To reply, replace the word 'spam' embedded in return address with 'mail'. N38.6 W121.4
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
I don't see how the car battery could possibly act as a voltage limiter, and thereby protect the delicate electronic and computer components. Assumint the welding voltage is going to be applied, somehow, to an electronic part, the only way the battery could have an effect would be if it were connected across the same part to ground--in parallel with the welding voltage. So you now have two low impedance sources with different voltages connected in parallel--a receipe for disaster. And, if you want to make the worst case scenario, picture the voltages being of opposite polarity!
So, if you're worried about welding NEAR your battery, take the darned thing out. And to repeat the earlier advice which I regard as very valid: Hook your welding ground cable as close as possible to the weld, so there is a direct path that does not include any other components.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
wrote:

Your mistake is that you are considering the battery only as a source of current. It is also a low impedance sink.
An auto battery (actually any rechargeable battery) acts as a very large and somewhat peculiar capacitor. It accepts charge from any source of higher potential and converts it to chemical energy. It releases charge at a fixed voltage. Therefore, it clamps any applied voltage to a value equal to its terminal voltage plus the IR drop across its internal resistance (which is very low for a starting battery in good condition).
If the applied voltage is reverse polarity with respect to the battery, the battery is also a large low impedance current source, and will supply up to hundreds of amps of current to prevent its terminal voltage from dropping. So regardless of whether the foreign voltage transient is plus or minus, the battery will try to hold the supply rails at its terminal voltage. This makes it a good voltage regulator or suppressor for transients of either polarity.
The battery *is* in shunt to ground (chassis) for all electrical devices in the car. And because it is a low impedance source and sink, it will hold the B+ voltage supplied to all those devices close to its terminal voltage, which not coincidently is the design voltage for those electrical components.
Now because its internal impedance is not zero, it has a finite RC time constant and won't do a *perfect* job of gobbling up all the transient energy. Very fast risetime transients won't be completely snubbed. That's why sensitive auto electrical parts have to have built in transient suppression sufficient to handle spikes up to about 200 volts. If you open up a car computer, you'll find ferrite bead chokes and shunt capacitors on all the power and signal leads. Those are there to knock down the fast risetime transients the battery can't suppress.
Gary
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
wrote:

Perhaps one other conseration is that during charging (or heavy discharge) a lead-acid cell will liberate hydrogen and oxygen -- which are capable of forming a very exposive mixture in a confined space.
Don't ask me how I can speak from first-hand experience but it involves a 9-year-old boy drilling holes in the top of a motorcycle battery so that he could light the hydrogen and watch it burn (fortunately I have no scars :-)
This is really just an academic risk however, since it's unlikely the battery will be gassing while you're playing around with the sparks and heat of welding -- unless perhaps you hook your earth clip up incorrectly and welding current passes through the battery.
-- you can contact me via http://aardvark.co.nz/contact /
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
: wrote:
:>First, realize that car electrical systems naturally produce transients :>as high as a couple hundred volts. So the on board computers, radios, :>etc have to have transient suppression built in to protect them during :>normal operation. Note also that the auto battery is a very good surge :>suppressor itself, so disconnecting the battery can be counterproductive.
: I don't know if hot welding sparks near a battery are ideal.. Maybe : replace the battery with a really large capacitor while welding on the : block near the battery?
Why not just short the positive lead to the ground? You really won't have any problem with the battery as it is not under a charging state to produce any H2 gas.
:>Meanwhile, your arc voltage is going to be on the order of 20 volts. :>Any voltage transients you produce are unlikely to be greater than :>those encountered in normal automotive operation.
: Could AC or HF start cause problems?
Just place the ground near the weld point. Try to avoid any bolted junctions between the weld and where you place the work clamp.
:>In the particular case you describe, I would unplug the oxygen :>sensor. It is a very low current circuit, and leaving it connected :>while welding on the manifold is just asking for trouble.
Tom
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

In effect, the battery *is* a very large and somewhat peculiar capacitor with the property that it tries to maintain its terminal voltage under all transient assaults. An ordinary capacitor won't do that as well. A very large capacitor and a *zener* would come closer to duplicating what the battery does.

HF could be a problem. It is thousands of volts, and at (obviously) a high frequency. The battery can't respond fast enough, due to its finite RC time constant, to high frequencies to provide adequate suppression. The car's electrical components have built in transient suppression for high frequency (fast risetime) spikes, but that's designed for a maximum of about 200 volts. Welder HF would overwhelm that protection. So I wouldn't recommend using HF when welding on a car.

Lightly melt filler into the surface of the part to form a base for the patch.

Well, yes and no. You want a good bond to the manifold, but you don't need much penetration to get that. You just have to achieve good fusion between the manifold and the bead.

Essentially yes.
Gary
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
wrote:

This I find interesting.
Now I know that it's traditional to use high-nickel fillers when fixing cast iron -- but I thought these would be specially formuated low-expansion alloys.
Given that stainless (well austenitic anyway) has a very high coefficient of thermal expansion (COE) compared to cast iron, using it to fix something like an exhaust manifold would be doomed to failure due to the massive sheer-stress set up as it heated (or cooled).
Whereas cast iron has a COE of around 6 and low-expansion nickel alloys are around 5.5, austenitic stainless is way up there at around 10.
The only reason this springs to mind is because I know how destructive something as simple as a thick-weld bead can be when putting a metal (like stainelss) that has a high COE through repeated thermal cycling to high temperatures and back.
-- you can contact me via http://aardvark.co.nz/contact /
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Disconnecting the battery is the most common response and is a great idea, but if you take care to 1. ground directly to the manifold (clean a spot) and 2. do not initiate the arc on anything but the manifold then stray current has no option to run through the pretty black boxes and let your factory installed smoke out. Patrick MTS
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
wrote:

Take the manifold off and weld from the inside too. If you leave the inner face of the crack exposed, even if you do manage a good weld from the outside, then you'll still have the crack propagating from the inside back out again. If you're a good welder it'll go round your bead, if it's a poor weld, then it'll go straight through it.
You'll also want the manifold off the engine, becuase you need to grind a hell of a vee in there. Do it without and it won't last a week.
Exhaust manifolds are a nighmare to weld. Best replaced.
Be especially careful welding turbo manifolds too. It's great fun when the weld fails and there's a free-range turbo now doing 30K rpm inside your dyno test cell, and nothing to hold it down.... 8-)
-- Die Gotterspammerung - Junkmail of the Gods
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
I have had great success on manifolds using a burning lamp to weld with a flux and cast iron rod, usual clean and deep vee then a peen with ball hammer after. have not failed after 12 years Brian

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Polytechforum.com is a website by engineers for engineers. It is not affiliated with any of manufacturers or vendors discussed here. All logos and trade names are the property of their respective owners.