Does the 1 liter 3 Cylinder need backpressure to run?

[kcg795]

I have a friend with a 91 Geo Metro. His exhaust was falling apart and something inside the exhaust collapsed making it so it wouldn't start. Now he has the exhaust system taken off. Now it starts. But it won't stay running and he thinks it's because there's no back pressure and thinks back pressure is needed for it to run. I have never heard of an engine needing back pressure to run. Any ideas? He's also replacing the entire exhaust. He's got some parts ordered now.

[geozukigti]

You can run that car open-manifold all day long if ya want. I would start digging deeper. Make sure all 3 cylinders are firing

[kcg795]

He did have it towed to a shop right when it quit working and they said the headgasket was blown and it was gonna cost $2000 to fix it. He told them to kiss his ass and had the car towed back home. He checked the oil and the coolant and even checked for bubbles while cranking the engine. His headgasket is just fine. But he's gonna see if the exhaust will fix the problem. I have a feeling it won't. Then he can start checking the spark. He's got new plug wires and new plugs. Cap and rotor and original though. I checked those when I replaced his plug wires and they were still in pretty good condition. If all else fails, he has another Geo Metro sitting in the backyard he can pull the engine out of.

[geozukigti]

I betcha the compression on the engine is wayyyy down. If the exhaust was as plugged as you say it was, it most likely wasn't releasing enough exhaust, the exhaust valves in the engine burnt up from the heat, and they're leaking. Probably just needs a head to fix her up. Here's what you need to do...
Get a compression tester
Check compression on all 3 cylinders
pour a little oil down into each spark plug hole to fill the cylinder with a small amount of oil (about 1 shotglass each)
run the compression test again
*recemmonded* only put oil in the cylinder you're testing, or oil will fire out the other open spark plug holes, and you'll be a mess

If the first test has any cylinders below 150psi, or not withtin 10% of both the other cylinders (EX: cyl1:170psi cyl2:140psi cyl3:168psi) you have a dead cylinder. What a new motor should read is 185 in each cylinder. 170 is an ok place to be. Anything below that, and it motor is gonna start running crappy, bad gas mileage, stall out, and the RPM's will drop a lot when you turn on the headlights.
Now, when you put oil in, and the compression jumps up on any, or all of the cylinders, the piston rings are shot, and you need to re-ring and hone, or new motor. If the compression is the same, you have burnt valves. In that case, you'll need just the head.

[kcg795]

I do have a feeling he's losing compression in one of the cylinders. He's getting spark to all of the cylinders, but it would always idle like it's missing in one of the cylinders until you'd rev it. He has the car in the shop right now and they're waiting for new parts for the exhaust system. He says Geo Metro engines aren't like any other engine and that they NEED backpressure to run. I still can't believe that. But if it decides to run after the new exhaust system is installed, then I'll believe it.

[geozukigti]

Nah, you can run a metro open headers all day long, if your ears like it. As a matter of fact, the less back pressure, the better it is for the valve seats. 0 backpressure will cause it to lose some power, but that's about it. All an exhaust system does is tune it to a specified power band, and keep the thing quiet. Tell him the Geo Master thinks he doesn't know what backpressure does for an engine

[kcg795]

If back pressure will make it run, then maybe it's because it's helping the compression a bit. He did get his parts and he decided that he's gonna install it himself tonight because the shop is taking way too long. If all else fails, I would like him to buy a compression tester and have the car towed here so I can try to fix it.

[geozukigti]

If your car needs backpressure to run, you'll be needing a garden hose to generate enough backpressure to make the burnt out valves seal. An engine's valves should seal with or without backpressure. As a matter of fact, when you install a 2" exhaust, and reduce the backpressure, it'll run far better, and more efficient. Compression is limited to the internals of the engine. Rings, valves, valve seats. Backpressure will not affect the compression. Your buddy is barking up the wrong tree

[kcg795]

Well. He called me up and said he installed the exhaust. He's missing the spring loaded bolts at the moment since the old ones snapped. But I dunno how he's got it hooked up at the moment. He's also getting a new exhaust manifold because the shop that was working on it was drilling out the broken studs for the heatshield and drilled right through the manifold. So he gets a new one for free. But he's got it jury-rigged so the car will now start. He took it for a drive and he says it hauls ass. He also said his old catalytic convertor was blowing chunks. He said his car never ran this good before. When I would ride with him and he'd take off, it sounded like riding inside a semi. I mean, it sounded like there was a turbo installed. But he said that sound is gone now. It sounds a lot smoother. There's so many things he told me over the phone that I can't remember. But he sure is proud that it's working a lot better than it was. He'll be able to drive it normally once he gets those spring loaded bolts and the new exhaust manifold. The spring loaded bolts are what bolts the manifold to the exhaust. He says they're like $47 a piece, which is a rip. But he's gotta have 'em.

I did think of something just now. Maybe it refused to run without the exhaust because the O2 sensor, if any, wasn't getting a very good reading and it was making his fuel supply shit out. Oh, and the fact that the catalytic convertor was blowing chunks and possibly clogging up the rest of the exhaust, the exhaust couldn't flow very good past the O2 sensor.

This is just my guess and possibly the reason why the car quit working when the exhaust system snapped off the manifold.

[geozukigti]

Heh, the cat was prolly a melted pile of metals. Those spring load bolts can be picked up at any auto parts store for $5-$7 each. Just the the GM bolts, they fit perfect. If he had the whole exhaust manifold off, maybe it was causing a vacuum leak for the EGR system. That would make it not start. The car should run, but get poor gas mileage without an O2 sensor.