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Erratic Engine Revs


Godsson314

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Hello! 

I have a 1995 olds cutlass convertible. The car rides smooth but after a harder acceleration (switching lanes on the highway) it can feel like it sputters. 

When coming to a stop at a light the revs are high and low (1.5 to below 1) sounding and feeling like it will cut off. I have the 3.4L. I switched out the IAC valve but that was a short fix, smells of gas in the garage if ran and getting horrible gas mileage. 

Looking for a great mechanic in San antonio if you know one!

Anyone have ideas? 

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Running rich could be all kinds of things. The car is dumping fuel in to balance out something wrong.  A good shop can still pull data from the ecm. That’s really the only way if you’re not going to wrench it yourself. Could be a bad O2 sensor., vac line leak, FPR is shot, even a bad temp sensor can make the ecm run the car rich.

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What is the fuel pressure?

How old are the usual "tune-up" items--plug wires, spark plugs, O2 sensor, verify PCV, EGR, and EVAP systems.  Check cranking compression pressure.  How old is the timing belt?

Connect a scan tool, verify EVERY sensor and computer output.  Check fuel trims.

O2 sensors get lazy with age.  If the O2 sensor has more than 50,000 miles or 5 years on it, it's probably due for replacement.  If there's a misfiring cylinder, it can drive the O2 sensor to report a false-lean condition.  That will drive the computer into rich-command.

 

Expect that the rich running has murdered the catalyst.

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16 hours ago, Schurkey said:

What is the fuel pressure?

How old are the usual "tune-up" items--plug wires, spark plugs, O2 sensor, verify PCV, EGR, and EVAP systems.  Check cranking compression pressure.  How old is the timing belt?

Connect a scan tool, verify EVERY sensor and computer output.  Check fuel trims.

O2 sensors get lazy with age.  If the O2 sensor has more than 50,000 miles or 5 years on it, it's probably due for replacement.  If there's a misfiring cylinder, it can drive the O2 sensor to report a false-lean condition.  That will drive the computer into rich-command.

 

Expect that the rich running has murdered the catalyst.

Question...how do you connect a scan tool to a 1995?

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1 hour ago, Godsson314 said:

Question...how do you connect a scan tool to a 1995?

I don't have experience with the older on board computers but a standard OBD2 scanner will not fit a pre-96 car. It's a different system.

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21 hours ago, Godsson314 said:

Question...how do you connect a scan tool to a 1995?

EXACTLY the same way you'd connect to any other vehicle with a diagnostic connector.  '95 is no different in that regard.

Find a compatible scan tool, use the correct adapter, connect it to the diagnostic connector.  You may need to supply "12 volt" power to the scan tool via a cigarette-lighter power adapter or similar.  Follow the instructions for the scan tool to type in the VIN number or whatever it takes to make the tool "talk" to the on-board computer.

 

Guys get all weird about pre-OBD2 vehicles and scan tools.  No need.  Just use a DECENT scan tool, and it'll work out just fine.  If you use a craptastic "code reader", or some consumer-grade bottom-feeder junk tool, you can expect it to be useless or worse, problematic.

Edited by Schurkey
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19 hours ago, Amanita said:

I don't have experience with the older on board computers but a standard OBD2 scanner will not fit a pre-96 car. It's a different system.

An "OBD2 scanner" doesn't work on a '95, because '95 is not OBD2.  Therefore, you'd need a scan tool compatible with what is now known as "OBD1".  They tend to be more expensive, and need a different adapter/connector for every brand of vehicle since there's dozens of proprietary connector bodies on the vehicles.  Every manufacturer used a different style of connnector.  GM alone had several depending on application.

A decent scan tool will work with GM products from 1980 1/2, onward to the limit of their software--often post-2000, perhaps all the way to 2024.  Mine goes up to 2007, but since I don't own a vehicle newer than 2003, I'm all set.

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