Engine stumbling

[TheStoot]

Hey everyone, just a quick question. I am going to change my plugs and wires soon but I have some engine revving issue. From an idle, if I stop the pedal down and rev it up (in neutral) the engine stumbles until i gets up past 2000rpm or so then it spins up great after that. If I slowly push the pedal down and let the rpm's climb slowly then there is no hesitation. I'm pretty sure my car is running rich because I still cannot get it to go into closed loop. Frequency valve doesn't buzz. Only does this at low rpm while driving also, once I get above 2-2.5krpm the engine is smooth as can be. Thoughts?

[Michael]

If it happened all of a sudden, check your vacuum lines. Have you recently been under the hood prior to this?

[TheStoot]

No it hasn't happened all of a sudden, it's been like this. I got to replace the original vacuum lines one of these days

[TheStoot]

There's no disconnected lines, but i can't imagine a small bad seal could cause this kind of noticable engine preformance decrease/hesitation?

[David T]

"Stomping" on the throttle and noticing hesitation in neutral is not a real test of anything. Do the tune-up. Check and if necessary change the air filter. Find and fix any vacuum leaks. Verify that all of the vacuum hoses are routed correctly, everything works, and no leaks. Check all of the throttle adjustments. Do a compression test and adjust the valves if necessary. Finally, check the A/F mixture and if necessary adjust the mixture screw. Make sure you get into "Closed Loop".

[Bitsyncmaster]

A wrong set mixture could do that. Get a dwell meter and check or set your mixture.

[dmcnc]

Also check your timing.

[TheStoot]

A wrong set mixture could do that. Get a dwell meter and check or set your mixture.

I can't check my mixture because the well just shows a set value because i can't get into closed loop

[Michael]

I can't check my mixture because the well just shows a set value because i can't get into closed loop

Well there is a pretty big red flag. Fix that first. Test and diagnose your o2 circuit.

[Bitsyncmaster]

I can't check my mixture because the well just shows a set value because i can't get into closed loop

If it holds (45 deg) with a warmed up engine than probably your O2 sensor is disconnected or bad. Another possible problem (holds 52 deg.) is the wide open throttle switch is always on.

[TheStoot]

If it holds (45 deg) with a warmed up engine than probably your O2 sensor is disconnected or bad. Another possible problem (holds 52 deg.) is the wide open throttle switch is always on.

I verified the WOT switch works with a meter (0 resistance when open) and I changed the 02 sensor out last week with a new one from D go

[TheStoot]

I put a new connector on the frequency valve and verified power at the plug, lambda ecu and idle ecu both have power

[CFI]

What is the dwell reading? It defaults to specific values depending on what’s happening. The frequency valve not buzzing can happen when the mixture is way off.

[TheStoot]

I got new plugs and wires on this afternoon. That cleared up the hesitation I was getting. Has more power at low rpms than ever before! I ran out of daylight and therefore didn't get the dwell never on but I turned the co screw more lean by a 1/4 turn to cut out the rich exhaust smell. Still no buzz from the frequency valve so ill have to do a bit more investigating this weekend. The plugs were AC Delco with the #16 on them

[Michael]

1/4 turn is a BIG adjustment! Messing with that mixture without a meter is asking for trouble. The new plugs may buy you some time till you foul them out again. Get yourself a dwell meter but until then you can use a multi meter which you probably already have laying around.

[David T]

If you are not getting not "closed loop" and the frequency valve is not buzzing and you were running over-rich, the O2 sensor is probably fouled. Replace it and see if that gets you into closed loop. By getting a dwell meter on and getting a reading, any reading, it tells you that at least the Lambda control is alive and working a little. As Michael said, 1/4 turn is a huge amount. If you were that far off your catalytic converter and muffler was probably glowing red, besides the overwhelming smell. Without a dwell meter the only adjustment you should attempt to the mixture screw is to set it so that you just shut off the fuel injectors. That only gets you close, about a 1/8 turn away from optimum, close enough for the motor to run without fouling out the plugs, O2 senor or converter. If the tamper proof plug is still in the mixture unit you should not touch the mixture screw. If the mixture screw hasn't been molested it is probably as close to perfect as you would get it trying to adjust it with a dwell meter. Not in your case if you already twisted it 1/4 turn. Do NOT adjust the mixture screw unless the fuel injectors are all spraying good patterns and equal amounts, there are NO vacuum leaks and everything is set perfectly. You CANNOT adjust away a problem with the mixture screw.

[TheStoot]

The mixture was messed with long before I got it. I am trying to fix what was done to it prior to get it running better. It has always run and started no problem, I drove it home 1400 miles before it had any tune up at all after 27 years sitting. It had all new fuel lines and fuel distributor and filter and such done, i just did the plugs and wires and o2 sensor and relays and fuses within the past week or 2. Whoever worked on it prior adjusted it to just get and keep it going. Now that I did the tune up I am leaning out the mixture and will put a dwell meter on it today to further correct it. I will post my dwell readings later this afternoon to see if I am going in the right direction with all of your help i hope!

[Jonty]

Looks like you are on the right track but don’t do too much messing with the mixture screw without a dwell meter. As others I’ve said 1/4 of a turn and you can be well past limits and all that happens is that you end up chasing your tail and fouling plugs along the way. Post the dwell readings when you can but don’t touch the mixture screw anymore until you do. Also make sure there are no vacuum leaks at all before tampering with mixture. Get a smoke test done if you can. Also how old are your injectors? Could be good to get them tested for peace of mind.

[steve]

Recently started in the heat of the south. 20069 idles high but smooth. Very slight pedal car will speed up ok exceed that very small pedal car bogs down hesitates until you floor it and hit the wot switch. This is in all
gears. Today temp. 75* started out drive as above. Fueled car all well on drive home.

Open for suggestions
Steve

[Bitsyncmaster]

Recently started in the heat of the south. 20069 idles high but smooth. Very slight pedal car will speed up ok exceed that very small pedal car bogs down hesitates until you floor it and hit the wot switch. This is in all
gears. Today temp. 75* started out drive as above. Fueled car all well on drive home.

Open for suggestions
Steve

Those symptoms tell me your mixture is lean.

[steve]

Thank you Dave will Re-calibrate.

Steve