Signs of CFI running lean

[91merc]

What are some signs of a CFI car running lean? I usually have to push on the throttle a little to get it started and sometimes it stumbles on the highway, I did a full tune up and metal timing set within the last year. I also average about 11-12 mpg

[Arquemann]

Some say having to give throttle to start is "normal", mine did that too. Mine also got shit MPG.
But mine ran rich.

How does it stumble on the highway? Under what circumstances?

[91merc]

Some say having to give throttle to start is "normal", mine did that too. Mine also got shit MPG.
But mine ran rich.

How does it stumble on the highway? Under what circumstances?

while I’m going down the road it’ll randomly just cut out and jerk momentarily

[Arquemann]

while I’m going down the road it’ll randomly just cut out and jerk momentarily

Hmm... Check plugs and spark, check your timing and how it goes depending on rpm.

[91merc]

Hmm... Check plugs and spark, check your timing and how it goes depending on rpm.

brand new plugs and wires, cap and rotor and we set the timing with a timing light

[kishy]

Throttle to start is required in many situations and is instructed in the owner's manual. There are some specific combinations of engine and weather temperature where it will start without throttle, but typically being into the pedal about a half inch is required especially when re-starting a hot engine that was recently running.

I chased misfire issues for a good long while on my 84 Town Car when I first got it, and the subject dominated most of my early presence on this site. The misfires presented as jerking/bucking (characteristic of running lean), typically when trying to maintain speed or gently accelerate when already at speed, but behaved alright from a full stop. I replaced pretty much everything except the engine and throttle body in pursuit of it. I did eventually cure it and it's been rock solid for 5-6 years since that now. I never identified what the exact silver bullet was, but I'd suggest the following:

• Electrical: verify your alternator (including its regulator) is behaving correctly. Ensure that there is good ground between alternator-engine, engine-body, body-battery, battery-ECM (factory provided 1-wire connector to negative post). Ensure ECM and fuel pump relay wiring are not full of green or black corrosion, and terminals aren't showing heat damage. Low voltage into the ECM and/or poor grounding will make it misinterpret sensor values and run like trash.
• Edit: there is also a thin ground wire that emerges from the engine harness and goes to a bolt on the back of the passenger side cylinder head. This wire is typically orange. I don't recall what it specifically provides ground for, but you should make sure it's present.
• Fuel: test your fuel pressure when cold, and test it again when hot. Even better if you can test it while it's bucking but that can be tricky with the test port's location. Target is 35-45 PSI and remaining stable.
• Ignition: if the wiring into the ignition module or coil looks crappy (and the coil wiring often does), consider replacing the pigtails. If your coil is old, consider replacing it. Insulation in coils can fail in ways that cause misfires or total lack of firing in specific temperature and humidity conditions, which can be a bear to troubleshoot.
• Check for codes. They are not always useful and can sometimes send you down the wrong rabbit hole, but it would be useful to know what the computer thinks is going on.

Your MPG sounds close to being 'about right' if you drive primarily in the city and with a heavy foot. About the best I can pull off in 100% city driving is 14 without significantly altering my driving style. If that average includes highway between 50-70mph, you've definitely got something up. But I'd figure that MPG values don't mean much when we know for sure it's misfiring.

If you're feeling bored and have a lot of time on your hands: https://www.grandmarq.net/vb/forum/t...ability-issues

[91merc]

If I turn my regulator up the ECM should keep the proper mixture shouldn’t it?

[91merc]

Throttle to start is required in many situations and is instructed in the owner's manual. There are some specific combinations of engine and weather temperature where it will start without throttle, but typically being into the pedal about a half inch is required especially when re-starting a hot engine that was recently running.

I chased misfire issues for a good long while on my 84 Town Car when I first got it, and the subject dominated most of my early presence on this site. The misfires presented as jerking/bucking (characteristic of running lean), typically when trying to maintain speed or gently accelerate when already at speed, but behaved alright from a full stop. I replaced pretty much everything except the engine and throttle body in pursuit of it. I did eventually cure it and it's been rock solid for 5-6 years since that now. I never identified what the exact silver bullet was, but I'd suggest the following:

• Electrical: verify your alternator (including its regulator) is behaving correctly. Ensure that there is good ground between alternator-engine, engine-body, body-battery, battery-ECM (factory provided 1-wire connector to negative post). Ensure ECM and fuel pump relay wiring are not full of green or black corrosion, and terminals aren't showing heat damage. Low voltage into the ECM and/or poor grounding will make it misinterpret sensor values and run like trash.
• Edit: there is also a thin ground wire that emerges from the engine harness and goes to a bolt on the back of the passenger side cylinder head. This wire is typically orange. I don't recall what it specifically provides ground for, but you should make sure it's present.
• Fuel: test your fuel pressure when cold, and test it again when hot. Even better if you can test it while it's bucking but that can be tricky with the test port's location. Target is 35-45 PSI and remaining stable.
• Ignition: if the wiring into the ignition module or coil looks crappy (and the coil wiring often does), consider replacing the pigtails. If your coil is old, consider replacing it. Insulation in coils can fail in ways that cause misfires or total lack of firing in specific temperature and humidity conditions, which can be a bear to troubleshoot.
• Check for codes. They are not always useful and can sometimes send you down the wrong rabbit hole, but it would be useful to know what the computer thinks is going on.

Your MPG sounds close to being 'about right' if you drive primarily in the city and with a heavy foot. About the best I can pull off in 100% city driving is 14 without significantly altering my driving style. If that average includes highway between 50-70mph, you've definitely got something up. But I'd figure that MPG values don't mean much when we know for sure it's misfiring.

If you're feeling bored and have a lot of time on your hands: http://www.grandmarq.net/vb/forum/te...ability-issues

at the moment I have a new Accel coil on it and all the wiring looked good surprisingly, it seems like it does the cutting out worse if I don’t let it warm up, especially in the winter. I primarily have a 10 mile one way to work and I’m usually driving 55-65 in that period

[91merc]

My alternator is strong enough that I was running my 91 GM off just jumper cables hooked straight to the battery leads and no battery in the 91 so I don’t think that’s an issue

[91merc]

Well this evening I hooked it up to a pressure gauge and it looks like whoever had it before me adjusted the fuel pressure to about 28#, I adjusted it back up to the 40# factory spec. The Allen head for the adjustment valve was a tad stripped out so clearly someone didn’t know what they were doing

[gadget73]

The orange ground wire from the ECM harness is the signal ground for the sensors screwed into the engine block, so oxygen sensor, coolant temp sensor, and air temp sensor. The three that have major effect on fuel mixture.

[91merc]

UPDATE:

After adjusting the fuel pressure back where it’s supposed to be, it starts way easier, much better driveabilty, and so far I’ve driven 22 miles and the needle hasn’t move off the F mark

[kishy]

Good catch. The regulator was probably turned down to compensate for some problem in the past; perhaps it was running rich and someone not understanding of how the system works thought that was the best way to deal with it.

As for the fuel gauge, there's a fair bit of tank above the "full" mark and I typically find I can go almost 100 miles before the needle comes down below F, but my cluster is different, being a Ford.

[sly]

I had the same with the 88 MGM I had. But it also has a dot below the E that the needle will point to when you're sucking fumes that's below the final hash mark that points at the E which notes the true empty spot on that car. The above F position also points at a similar dot on that end as well.

[gadget73]

my Towncar will do about 75 from the top peg down to F. E is absolutely E though.

Interesting that someone knew enough about the system to dick with the pressure, but not enough to actually figure out what was wrong with it. Its not that uncommon with old stuff to find people have messed with things to try and work around a problem and end up introducing new issues because of their "fix".