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kmpdx

Question about diagnosing restricted exhaust

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1991 SPFI manual Loyale w/212k miles

 

So I am waiting for a pressure gauge to come to test the vacuum, I am still driving it to try to diagnose.

 

It never really has full power but I noticed in different gears on flat ground I get to certain RPM/speeds.

 

1st gear 5k+RPMs 20 mph

2nd gear 4k RPMs 30 mph

3rd gear 3k RPMs  40ish mph

 

When it hits the max RPMs, the gas cuts even if I floor it and only will come back on once I take the gas off and let it go below the RPMs listed and then it will get gas and cut out again relatively consistently with the numbers I posted above. It never has the power I know that it should in any gear and the RPMs never match the power I expect like what I had when it was functioning properly.

 

Does this still sound like resricted exhaust? 

 

I will check pressure and or pull O2 sensor to see if I get the same results next weekend.

 

Thanks,

kmpdx

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After driving home again I don't think that the gas actually "cuts out" but that regardless of how hard the accelaerator is pressed the power prdoduced maxes out at the above listed RPMs. I suppose if that were true that it would support the resricted exhaust.

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Restricted exhaust will usually occur at a set rev or throttle load.

 

For instance the peak speed will drop when climbing a hill due to more throttle required to get up said hill.

 

You can also gently tap the exhaust sections to listen for internal rattles as this can indicate a blockage. The cat would be most suss as these can break down and restrict performance until they have the guts blown out of them if it ever gets to that point.

 

Cheers

 

Bennie

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Plugged fuel filter, incorrect timing or a plugged air filter would do the same. Bad MAF maybe. Any codes?

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I tapped the rear cat and no rattle but I have not yet tapped the front cat. I do suspect that is where the problem is if it turns out to be restricted exhaust but will need to get down there next weekend. I will also pull the O2 sensor at that time and see if that makes a difference. Having the guts blown out is what the guys at the auto parts store suggested. How should I do that? 

 

Fuel filter is good, Air filter is good, MAF looks good and was cleaned using the right cleaner in the last year although there was a normal amount of oil vapor in the intake boot. When you say incorrect timing are you talking the timing belt? It is old and should probably be replaced. It did happen all of the sudden under load on the freeway. So maybe it did jump a tooth? Does that fit this picture? 

 

I will explore those options once I can definitively rule out restricted exhaust.

 

Thanks,

kmpdx

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It could have jumped a tooth. Check it and the ignition timing. Pulling the O2 to drive around and test for a plugged CAT won't do anything. The hole isn't big enough to relieve enough pressure to make a noticeable difference if it's plugged.

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So I would like to test for back pressure. One poster suggested that I test for back pressure using the EGR? Can somebody explain this to me? Is this the best place to connect it to?  I really would like to rule this out if I need to. Then  I can try other things. I have a infrared tester but I don't have it here so have been reluctant to buy another one here. 

Thanks,

kmpdx

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An infra red thermometer is not going to tell you if it's plugged or not. It's only going to tell you if it's lighting off or not. Just because it's not lighting off doesn't mean it's plugged. You need a back pressure tester. Remove the O2 sensor and install the tester. I've never heard of anyone testing back pressure at the EGR, nor have I ever seen a tool to hook up the tester there. Not to mention, exhaust gas in the exhaust pipe is actually under a vacuum whereas at the EGR, it's under pressure so I don't know how you'd get any accurate info out of that. You need to set it at top dead center and check the timing belts and the ignition timing. CATs don't plug up all of a sudden. It happens over time. My money is on a slipped timing belt.

Edited by skishop69

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Not to mention, exhaust gas in the exhaust pipe is actually under a vacuum whereas at the EGR, it's under pressure so I don't know how you'd get any accurate info out of that.

No, the exhaust system is not under a vacuum, it is under pressure from combustion cycle. The intake is under a slight vacuum skishop69.

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No disrespect to the OP, but you're all over the place with this diagnosis stuff, kmpdx. And so are we in a way.

 

When you finally do get some solid info, come back & post it for us to help you if you still need it.

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So basically I don't mind learning something new and doing my own research. I am not an expert and in the process of learning. This issue is pushing the limit of my knowledge which I will admit isn't extensive. When I was living in Portland, I could bounce things off my local mechanic which was super helpful but I am in a nursing program in southern Oregon and don't have a relationship with a mechanic here so I am relying on you guys and Google/Youtube. I appreciate your help, but if it is not specific enough for you, you may choose to check back later when I do get more specific information. 

 

I did get a vacuum gauge. While it is not a definitive test, I did not get the the steady drop in vacuum that I would expect when accelerating the motor to around 2.5k RPMs if it were a restricted exhaust.

 

If I pull the O2 sensor, What size socket would I use? I did try to get a deep 22mm socket in there but I need to jack up the car I think to get it in there. Is 22mm the right size?.

 

Thanks,

kmpdx

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As someone previously suggested  in another thread  regarding this same problem -  disconnect exhaust temporarily - ie I suggest at end of Y pipe  (the engine side of cat.)  -  to see if it runs better.

 

Need to systematically go through stuff and eliminate each possibility.

Edited by subnz

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Not to mention, exhaust gas in the exhaust pipe is actually under a vacuum whereas at the EGR, it's under pressure so I don't know how you'd get any accurate info out of that.

No, the exhaust system is not under a vacuum, it is under pressure from combustion cycle. The intake is under a slight vacuum skishop69.

Incorrect. You need to hook up a back pressure gauge and watch it. At idle and most throttle conditions, exhaust is under vacuum. As the hot exhaust gasses go through the pipe, they cool and become more dense and pick up speed. This causes a draw on the system creating a vacuum. It's only 1"-2", but it is a vacuum. The only time it may be under pressure is during hard acceleration and even then, it should not exceed 2psi and ideally should be 0psi. Any other lifetime mechanic here will tell you the same. When emphatically correcting someone. you should be certain of your facts before doing so.

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The o2 sensor is 22mm. You need a special o2 socket to remove it without cutting the wire, or an open end / box end wrench.

 

Unless it's been recently replaced or serviced, it is likely very stuck, the special socket.might not be able to turn it.

 

The o2 sensor is only used to fine tune the mix, once the engine is at normal operating temperature, it won't be the cause of the trouble.

 

The hole is too small to make much difference if the exhaust is partially blocked.

 

The time I had a blocked catalytic it wouldn't get anywhere near 3000rpm, and struggled to move the car.

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Hey Dave T. I did cut the wire and put a connector so that I could disconnect it the O2. My real issue is having time, space and tools to actually do the troubleshooting. I am going to try this weekend. 

 

I don't really have the tools I need here drop the exhaust.

 

I think my next tests would be timing and compression to try to do some general rule out but may have to bite the bullet and take it to a local mechanic next week. 

 

Either way, I will report back here with any findings.

 

Thanks,

kmpdx

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Yeah, time space and tools, or shortage of same can be troublesome.

 

I can only go by what I've run into / done / etc.   For example, I have a vacuum gauge, and know they can be used to determine a number of problems, but I never used it to verify an exhaust problem.  For one reason or another, all of my EA82s at some relatively early point, I removed the 4 header bolts, and put anti seize on them, so loosening the 4 bolts for the exhaust test is simple and direct.  If yours have never been touched, or haven't been in a long time, you'll want the time space and tools to deal with it.

 

For timing, with a light, IIRC, you need to connect the single pin green test connectors.  20 BTDC

 

Good luck, let us know what you find.

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