Surging idle on cold start

[1997]

Ah, I see. Can you freeze frame? That is, hit a button to capture the data at a particular point in time? If so, maybe comparing the data captured during the idle surge vs non-surge may show something? Eg. A burst of knock counts, some sensor data changing, etc?

unfortunately not for OBD 1, but it does a bunch of frames every few min's when things change (in my pic's its showing frame 0 and i have 11 frames) so i should be able to review.
thx for your help.

Bodageta1995 my cel trim changes to 5 at about 1200rpm.​

im wondering if your thermostat is stuck open.

appears to operationing correctly via gauge and scanner.

 

[Bodageta1995]

are you getting a constant code 44?
no codes were present on my posted live data display, and issue is intermittent, but getting more frequent. we have the same 95 TBI 5.7, mines a 6 lug K2500.
i'll post up some open loop info when i get it and read the manual for both truck and scanner some more

I haven’t cleared the code 44 to see if it comes back. I should. Mine seems O2 sensor rich after the cold start during stumble but lean overall based on fuel trims. If I go wide open throttle, when it’s warmed up and running smooth, it’s a constant lean O2 but if I back off the O2 sensor starts cycling. That makes me wonder if the fuel pump is weak. I’m wondering how the fuel pump if weak could give a rich condition at start. Maybe it’s over compensating. Fuel Pressure check and homemade smoke machine test for vacuum leaks is in my future.

 

[Schurkey]

If I go wide open throttle, when it’s warmed up and running smooth, it’s a constant lean O2 but if I back off the O2 sensor starts cycling.

I had a similar problem, many years ago. Lean code at highway speed, lower speed was just fine.

I fought that for two years. Looked at all sorts of stuff--fuel pressure, AIR system, ignition timing, O2 sensor cross-counts, on and on. Nothing seemed wrong when tested, but the code would re-set reliably when the speed went up.

Finally, in desperation, I just replaced the O2 sensor.

Problem solved.

 

[Mach 5]

Here's my surge story. (1990/K5/383) Started fine, after idling just fine for a few minutes it would start to surge, to the point it would stumble so badly it nearly stalled. But then, after another minute it would smooth out again. Second verse, same as the first. After no codes and checking everything twice, three times, it turns out the stupid little connector on the top of the temperature sensor was jiggling itself loose, and then not, occasionally, randomly, sending intermittent signals resulting in surging. Fixed the connector, solved the problem. (Miller time!)

 

[1997]

update, more testing. Definitly worse as temp drops.

nutshell version reported by scanner, when surging starts, it reports flipping back and forth between open and closed loop.

cold start, open loop, warms up a bit, no surging, switches to closed, stays that way for about a minute, then surging starts and switches back to open loop, then closed, back and forth for about 2 minutes, then surging stops, closed loop good to go.

What sensor(s) decide when it's open loop or closed loop? (temp, 02? ..)

thx.

 

[Schurkey]

Mostly O2 sensor. There's also the potential for time the engine has run, and coolant temperature. In other words, open-loop until the engine has run X amount of time, the coolant temp is at least YYY number of degrees, and the O2 sensor is hot enough to provide usable output. When all the conditions are met...computer switches to closed-loop.

Look at O2 voltage on that scan tool, and also the coolant temp reading. If coolant temp is stable, you're maybe looking at faulty O2 sensor, or faulty O2 sensor wiring. If the coolant temp is not stable, faulty coolant sensor or wire harness.

Outside chance of faulty ECM.

 

[1997]

02 sensor = open loop = 1101 mv
02 sensor = closed loop = 200 - 1000 mv

Temp sensor appears to be stable, goes up slowly as it warms up, no fluctuations.

appreciate your help.

 

[Sabinoerc]

then surging starts and switches back to open loop, then closed,

Just to confirm, it only surges when in closed loop?

there is a check on how fast the O2 is responding to a change in A/F commanded by the ECM. I had a “lazy” O2 once where it was getting to max/min ( I think it has to get below 0.4V and above 0.8V or 1.0V, can’t remember) but not fast enough and it would drop out of closed loop. I didn’t have surging in closed loop, just caused me to fail emissions. O2s need to be at temp to work well so maybe it’s too slow until it gets hotter? Hard to say but if you haven’t replaced the O2 you might do that to eliminate that as variable. Not too expensive and easy to get to.
Ive read some people replace O2 with 4 wire heated sensor but I have stock cast exhaust manifolds so just use the stock 2 wire.

 

[1997]

Just to confirm, it only surges when in closed loop?

there is a check on how fast the O2 is responding to a change in A/F commanded by the ECM. I had a “lazy” O2 once where it was getting to max/min ( I think it has to get below 0.4V and above 0.8V or 1.0V, can’t remember) but not fast enough and it would drop out of closed loop. I didn’t have surging in closed loop, just caused me to fail emissions. O2s need to be at temp to work well so maybe it’s too slow until it gets hotter? Hard to say but if you haven’t replaced the O2 you might do that to eliminate that as variable. Not too expensive and easy to get to.
Ive read some people replace O2 with 4 wire heated sensor but I have stock cast exhaust manifolds so just use the stock 2 wire.

surging appears to start when its switching between open and closed loop.
open loop ok, closed loop ok once its a warmed up a bit.
thx for you input.

 

[1997]

back on the hunt to repair this.
unplugged CTS, won't start, set code 15, plugged back in.
unplugged 02 sensor, same issue, no code set, scanner reported no closed loop and 02 sensor reported RICH.

Does this sound right?