16 MPG?! Smells like gas fumes all the time?

[darklotus42]

So 1st off- 95 CS 3100 144k

And as the title says I'm only getting about 16mpg. And while it's running and shortly after shutting it off there is a strong odor of unburnt gas fumes. Could this be related to my EGR or EVAP system? My live stream data says the EGR solenoids apparently never cycle. Also I'm getting a stumbling with during acceleration that I'm assuming could the be coil packs altho I still need to check the vacuum system. Any thoughts how to fix?

[Human]

I have a similar phenomenon with my '95 CS Convertible, but raw gas odor mainly manifests when I first fill the tank, and is usually gone by the time I've driven 20-30 miles. Fuel economy, as near as I can estimate, averages around 18mpg, but the engine runs smooth as silk, no stumbling at all. Also, filling the tank can be tricky, as it doesn't always automatically shut off the pump. When it fails to shut off, gas will eventually spill out onto the ground from under the passenger side, opposite of where the filler is. It does not overflow from the filler neck itself. My (admittedly imperfect) solution is to put 10 gallons in when the gauge falls just below 1/4 tank. I'll put up to 12 gallons in, if the needle is down in the red one, but I never try to top it off.

[darklotus42]

Yeah that is weird. However on mine it isn't the odor of raw gasoline. Its similar to how it smells with a rich mixture but I would figure you wouldn't be able to smell that after the car has been shut off. Somebody suggested I could have plugged catalytic converter. I know I've bottomed out a few dozen times (bad suspension) so maybe that's the problem?? Idk I just wish I could hook a scanner up and it would tell me but OBD 1.5 sucks

[Schurkey]

10 hours ago, darklotus42 said:

95 CS 3100 144k

 I'm only getting about 16mpg. And while it's running and shortly after shutting it off there is a strong odor of unburnt gas fumes. Could this be related to my EGR or EVAP system?

Not EGR.  Could be EVAP.  More diagnosis needed.  At 16 mpg, you can probably bet that your catalyst is ruined.

My live stream data says the EGR solenoids apparently never cycle. Also I'm getting a stumbling with during acceleration that I'm assuming could the be coil packs altho I still need to check the vacuum system. Any thoughts how to fix?

The Usual Three:

1. Verify fuel pressure at prime, at idle, and under load. Most fuel pressure gauge assemblies have a push-button pressure release connected to crappy vinyl tubing. Route the tubing so it empties into a drain pan, then push the button while the engine is running. This simulates higher fuel demand if you can't check fuel pressure on the highway. Fuel pressure should remain steady even with fuel flowing down the pressure-relief tubing. TBI systems don't hold pressure when the pump stops running, but the other fuel injection styles should. How old is the fuel filter? Have you ever dumped a bottle of Chevron Techron Complete Fuel System Cleaner into the gas tank? (Recommended at every oil change.)

2. How old are the usual “tune-up” parts and procedures? Inspect/replace plug wires, spark plugs. Use quality parts sourced from an authorized seller (NO COUNTERFEIT PARTS FROM AMAZON, EBAY, OR OTHER SKETCHY SELLERS!) Be sure the ignition coils will reliably fire a spark-tester calibrated for HEI when the coil is fully warm, and misted with water from a squirt-bottle. Cranking compression test of all cylinders while the spark plugs are removed. Verify EGR, PCV, EVAP, and Heated Air Intake (if used) systems for proper operation. Verify proper electronic spark advance. Replace old O2 sensors unless you can PROVE that they're working properly—old O2 sensors get lazy, they don't provide accurate data, but they do provide “data” that fools people into thinking they're “working”.

3. Connect a scan tool (NOT a crappy “code reader”) and look for “codes”. More important, look at the data stream to verify EVERY sensor and computer output. Verify fuel trims during the time that the vehicle is not running properly. “Codes” have official diagnostic procedures that will be found in the service manual set for your vehicle.  1995 "OBD 1.5" is NOT a problem for any REAL scan tool.

[Vegeta]

A cat would be plugged from poor operating conditions, and you would have no power.  See if the fuel pressure regulator has fuel in the vacuum line.  Thats my shot in the dark.  its not gonna be evap at 16 mpg.  You have either bad ignition, stuck injector, bad fuel pressure regulator.  See what your BLM/INT are at.  LTFT and STFT would be the same thing, fuel trims.  

[darklotus42]

What kind of parameters should I look for?

[Vegeta]

Fuel trims.  I dont know your computer to say if its INT and BLM or if its labeled STFT and LTFT.  int/blm would be 128 nominal, and LTFT and STFT are % based from 0% fuel trim.  

[darklotus42]

So the other day I noticed my rear floor board was really warm so I looked underneath and sure enough my cat was glowing. I was considering gutting it but I changed my plugs and wires and that helped A LOT although I'm still getting a stumble under acceleration. No codes. Here's some live stream data numbers related to my fuel system. The commas indicate Idle/low throttle/ high throttle. 

 

LT FUEL TR CL 16,4,0

LT TRM R 131

KS 29

IAC 60-75

IDLE 863-913

INJ PW 4, 9, 13, 0

ST FUEL TRM R 130

MAP 2.3, 1.9, 3.5, .3

MAP HG 14.8, 24.6, 9.4

IAT 169

O2S R 870, 170, 85

QUAD 4 BAD

SPARK ADV 25, 15

TPS .6-1.2

Edited January 3 by darklotus42

[rich_e777]

Gut that thing! Even when cat converters are converting cats at peak performance, its still negligible benefit to the environment. They've been a virtue signaling gimmick since the late 90s when engines like the 3100 where doing just fine with emissions without them.

[bluecalais79]

Hello Rich, is there an exhaust system for the 1994 CS 2 door 3.4 you can recommend that will eliminate the Cat Converter? Also, I'd be looking for shorter mufflers too, I'd like to have the ADDCO rear sway bar installed and right now I can't since the bar is so thick it will hit the exhaust clamp. I live in SC and they do not require emissions testing so that is not a concern. No one cares about 30 year old cars here anyway. No extra noise though, factory 3.4 growl is just fine. 

Regarding the smell of gas, ever since I had my gas tank & gas tank neck replaced, I have a real issue with the fuel pump shut off not working at all. I learned the hard way the first time I filled up. So, I use the same method when I fill up. Basically I calculate how much fuel I need to put in based on how much I used via reading the gauge. An example is if it reads 1/2 empty I put in 8 gallons. 

Edited January 10 by bluecalais79

[Bake82]

3 hours ago, bluecalais79 said:

Hello Rich, is there an exhaust system for the 1994 CS 2 door 3.4 you can recommend that will eliminate the Cat Converter? Also, I'd be looking for shorter mufflers too, I'd like to have the ADDCO rear sway bar installed and right now I can't since the bar is so thick it will hit the exhaust clamp. I live in SC and they do not require emissions testing so that is not a concern. No one cares about 30 year old cars here anyway. No extra noise though, factory 3.4 growl is just fine. 

Regarding the smell of gas, ever since I had my gas tank & gas tank neck replaced, I have a real issue with the fuel pump shut off not working at all. I learned the hard way the first time I filled up. So, I use the same method when I fill up. Basically I calculate how much fuel I need to put in based on how much I used via reading the gauge. An example is if it reads 1/2 empty I put in 8 gallons. 

There will be no off the shelf exhaust to remove the cat.  You'd need to find an exhaust shop to do it for you, or cut and clamp yourself.   

[Schurkey]

It's been a long time since I last checked.  Back then, there was a $10,000 FEDERAL (not State, not local.  EVERYWHERE in the USA) fine for an exhaust shop to bypass a catalyst.

The only thing you have going for you is that the Feds don't often enforce the penalty.

If you don't like offensive odors, you're gonna hate not having a catalyst.

Good luck.

Edited January 12 by Schurkey

[rich_e777]

On 1/10/2025 at 8:55 AM, bluecalais79 said:

Hello Rich, is there an exhaust system for the 1994 CS 2 door 3.4 you can recommend that will eliminate the Cat Converter? Also, I'd be looking for shorter mufflers too, I'd like to have the ADDCO rear sway bar installed and right now I can't since the bar is so thick it will hit the exhaust clamp. I live in SC and they do not require emissions testing so that is not a concern. No one cares about 30 year old cars here anyway. No extra noise though, factory 3.4 growl is just fine. 

Regarding the smell of gas, ever since I had my gas tank & gas tank neck replaced, I have a real issue with the fuel pump shut off not working at all. I learned the hard way the first time I filled up. So, I use the same method when I fill up. Basically I calculate how much fuel I need to put in based on how much I used via reading the gauge. An example is if it reads 1/2 empty I put in 8 gallons. 

Like what Bake82 said, find a GOOD exhaust shop to eliminate to converter, could be done with a piece of pipe. I have a pair of mufflers I bought off a guy with a 2nd gen GTP for the C5 tips. You can have them if you want them, IIRC they`re Thrush brand.  

I was able to snag a high flow cat converter assembly that had a new downpipe attached to it and bolted up to the flange. It went into a single in dual out muffler right under the center console area and then straight piped to C5 tips. Love the sound but the guy went under the monoleaf and lat links so the exhaust hangs a bit lower than I`d like. I had a set of glass packs in place of the factory mufflers before and it was way to loud. This setup reminds me of a more sportier Jaguar or BMW, well when the engine was in it back in 2020 it did.

[Schurkey]

On 1/3/2025 at 2:47 PM, rich_e777 said:

Gut that thing! Even when cat converters are converting cats at peak performance, its still negligible benefit to the environment. They've been a virtue signaling gimmick since the late 90s when engines like the 3100 where doing just fine with emissions without them.

Have you ever crammed an exhaust-gas analyzer into the tailpipes of no-catalyst vehicles and catalyst-equipped vehicles?  Kinda guessing "no".

[bluecalais79]

Thanks Rich, I'll pass on the mufflers, for now I will keep the existing original exhaust until it's ready to drop on me. 

[rich_e777]

10 hours ago, Schurkey said:

Have you ever crammed an exhaust-gas analyzer into the tailpipes 

Are we still doing phrasing because you know full well I have not,

A. because I`m a believer in global warming and I freakin hate the cold.

B. because you have yet explained why cat converters should be required other than it makes a funny smell. 

C. because they dont make catalytic converters in the US anymore. Why should I help funnel US dollars overseas into the hands of those that determined me a servant class human in their NWO agenda? When I really do not "have" to because they say?

I would love to here this(insert My Cousin Vinny gif)

[Schurkey]

5 hours ago, rich_e777 said:

A. because I`m a believer in global warming and I freakin hate the cold.

I believe in Global Warming in that the Seasonally Frozen Wastelands no longer get cold enough to get rid of riff-raff.  Whether PEOPLE have caused that, is still debatable.

[Ooops.  Lost a quote here, and I'm too lazy to find it again.  It was your "B" point--"because you have yet explained why cat converters should be required other than it makes a funny smell. "]

1.  Because they're effective in reducing tailpipe emissions

2.  Because they eliminate the stench of hydrocarbon emissions

3.  Because the high-flow, monolithic cats aren't restrictive like the original GM pellet-style.  The only benefit of pellets over honeycomb is that the pellets could be removed and replaced--making that style of catalytic converter "rebuildable"

4.  Because it's Federal law.

5 hours ago, rich_e777 said:

C. because they dont make catalytic converters in the US anymore. Why should I help funnel US dollars overseas into the hands of those that determined me a servant class human in their NWO agenda? When I really do not "have" to because they say?

I don't know about the manufacturing of cat converters.  If you say they're not made in the US, then I agree that's a shame.  Whole swaths of American industry have been wiped-out by the Communists, and the Communist Collaborators in Industry and--especially--Government.

I'm more concerned about the hollowing-out of the Medications industry and the Electronics industry than the exhaust industry--but all are a problem.

Edited January 15 by Schurkey

[rich_e777]

Sorry OP for the derailment, a long time members quote still rings true. Ignore 99% of the BS you read here but ALWAYS listen to Schurkey.

 

@SchurkeyI hope you understand I was just poking the bear a little bit and always enjoy reading your posting style. We`d get along just fine so long as there wasnt a throwable/swingable object within your reach. 

 

[Schurkey]

We're good.

[GnatGoSplat]

Did you check the coils?  Symptoms sound like classic coil issues.  Especially if there's an odor.  To me, the odor smelled like burnt matchsticks.

Overly rich exhaust can make the cat glow.

Could be clogged cat too, but coils are more common.  I use an old school inductive timing light to check them.

 

[Schurkey]

On 1/26/2025 at 11:10 PM, GnatGoSplat said:

Did you check the coils?  Symptoms sound like classic coil issues...   ...I use an old school inductive timing light to check them.

A timing light can tell you that there "is" a spark, and would give a false-negative if the spark plug is fouled.  WAY more effective to use a spark-tester calibrated for HEI which also tells you that the spark is strong enough while removing the vehicle spark plugs from the test protocol.  There's multiple vendors, and various styles.  My preferred style is like this one:

https://www.amazon.com/dp/B003WZXAWK/?coliid=I3S98D7T1J0RLJ&colid=2VLYZKC3HBBDO

Edited Wednesday at 05:21 AM by Schurkey

[GnatGoSplat]

8 hours ago, Schurkey said:

A timing light can tell you that there "is" a spark, and would give a false-negative if the spark plug is fouled.  WAY more effective to use a spark-tester calibrated for HEI which also tells you that the spark is strong enough while removing the vehicle spark plugs from the test protocol.  There's multiple vendors, and various styles.  My preferred style is like this one:

https://www.amazon.com/dp/B003WZXAWK/?coliid=I3S98D7T1J0RLJ&colid=2VLYZKC3HBBDO

Thanks, I need to get one of those, especially now that I have 2 cars with coil-on-plug that an inductive timing light won't work.

 

Edited Wednesday at 02:15 PM by GnatGoSplat