2nd Generation Specific 1986-1992 Discussion

Exhaust upgrade causing low compression?

Old 03-23-2011, 03:34 AM
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want to know if for the 2nd gen Turbos un-restricting the exhaust back-pressure can in anyway make the car lose compression or flood.

I have an 88 Turbo FC with a custom hi-flow cat (the pre-cat is drilled out) and an extra wide single pipe going to a Racing Beat stainless steel rotor-shaped exhaust.

123k miles. I've taken it to dealer to diagnosis and they said the fuel-injectors were running fine, but the engine itself had low compression and advised me to replace the whole thing. They didn't give me any compression numbers either... but when I took the spark-plugs out and cranked the seals sounded completely uniform. The problem is that it Needs a fuel-cut-switch to restart after warm-up, it gets strong gas smell in the oil and exhaust and only gets 16 mpg. Plus it's boggy at low rpm.



Short of a rebuild, I was just wondering (and didn't see this covered anywhere else) if having such a wide exhaust could in any way cause these symptoms or if it's just probably the rotor housings that are out of spec?



Thanks---
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Old 03-23-2011, 08:15 AM
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Exhaust should have no material influence on the compression numbers. New injectors or properly cleaned injectors would likely make the hot restart issue better. At 123k miles, your compression is likely a bit on the low side, and that will make the hot restart/flooding problem worse.



The fuel cut-off switch is another way to lessen the problem of flooding, but it doesn't address the root cause.



If you paid the dealer for a compression check, I would damn sure ask them for a copy of the results. Unless there's a big difference from one rotor to the other, or if the numbers are really low, as in below 80, I wouldn't be too concerned. Drive it until it quits if you can afford the gas.



Does it smoke?
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Old 03-24-2011, 02:17 AM
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Originally Posted by 1988RedT2 View Post
Exhaust should have no material influence on the compression numbers. New injectors or properly cleaned injectors would likely make the hot restart issue better. At 123k miles, your compression is likely a bit on the low side, and that will make the hot restart/flooding problem worse.



The fuel cut-off switch is another way to lessen the problem of flooding, but it doesn't address the root cause.



If you paid the dealer for a compression check, I would damn sure ask them for a copy of the results. Unless there's a big difference from one rotor to the other, or if the numbers are really low, as in below 80, I wouldn't be too concerned. Drive it until it quits if you can afford the gas.



Does it smoke?


Does not smoke. Just a rich fuel smell in the exhaust.

A previous mechanic pulled wave-forms off the fuel-injectors and said they checked out fine.

But again, they did not provide me with the numbers. I asked for them and they said they didn't have them "recorded". So I have no idea whether I should believe them or not.

Finding people to work on Rotaries is Hard in Colorado.



Anyway- that answers my question if back-flow affects compression, so thanks.
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Old 03-24-2011, 06:22 AM
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Pulling the wave forms off the injectors will tell you if they're getting the proper signal from the ECU, but it won't tell you if the injectors are clean enough to keep them from dripping gas into your intake when the car is shut off. I had a persistent hot start issue with my T2 until I removed the injectors and had them cleaned.
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Old 03-24-2011, 07:14 AM
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Get a mechanic familiar with rotaries to give you a compression test. These engines can and do wear, and show low compression without any serious smoking. And get the numbers, dont just take low or fine for an answer. 120 psi is good, anything below 95 and you should be looking to have it rebuilt, unless you have heavy porting that affects the compression numbers.



On a side note, an open exhaust makes starting with low compression easier, not harder.
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