**Resolved** After replacing the O2 sensor and other things on my 1989 K5 its lugging and idling poorly

[clutch2k3]

tl;dr Root cause was bad spark plug wires AND crossfire.

Original post:

Any help is appreciated and feel free to ask me any questions. I attached data logs.

My K5 is experiencing issues after performing some work on it this summer

• Hunting and "acting funny" at idle. The IAC is only opening between 0 and 11 steps in closed loop.
• Very noticeable lugging/surging during TCC lockup up in 3rd & 4th on flat roads and really really bad lugging/surging on inclines during TCC lockup up in 3rd & 4th
• TCC locking up at 800 925-1000 RPM in 4th
• Its shifting into 4th at only 35 MPH

Before performing the work below, the truck wasn't experiencing any issues

• replaced the stock manifolds with summit headers (had to remove/reinstall spark plugs & wires)
• replaced the O2 sensor
• Repaired the O2 wire at the back of the engine (the previous owner or muffler shop hacked it up pretty badly)
• fabricated a new 2.25" exhaust
• installed 2" lift springs
• replaced the trans send line with braided AN-6 and added an AN-6 tee & adapter for the trans temp sender (the compression fitting on the hard line started weeping)
• Added a TCC lockup override switch
• replaced the muffler with a flowmaster 70 series truck muffler
• deleted the cat

Here’s the troubleshooting I’ve performed so far

• Bypassed my TCC lockup override switch (no change)
• Verified the TV cable isn't loose
• Drove it 4 days for the ECM to relearn fuel trims
• Overriding TCC lockup helps smooth out the lugging/surging and I haven't seen the trans go over 170°F
• Verified the sensors look OK in WinALDL
• Sprayed brake cleaner all around the intake manifold to check for vacuum leaks but there's no change in idle
• Performed the IAC initial calibration twice
  • Adjusted RPM w/throttle blades to 500 RPM 1st time and 450 RPM the second time
  • Cleared the BLM table and SES codes by disconnecting the battery overnight
• Tried idling with the AC on and the blower on max, adding load via the alt, and the IAC opened 30-50 steps and the idle settled
• Re-verified all fluid levels
• Collected data logs to look for any differences in and out of TCC lockup and during idle

What I saw in the logs

• The ECM is *pulling* fuel the entire time its lugging/surging
• There are no knock events during the lugging/surging
• CTS reads the same in closed & open loop
• When I swapped the new O2 with the old O2
  • It idles OK in open loop w/the old O2 but it wasn’t cycling between 0.1v and 0.9v, it was just hanging around 0.5v. After taking the truck on the road, it went into closed loop, it drove just about the same as with the new O2. I noticed the ECM was going in and out of closed/open loop with the old O2. While the ECM was in open loop it drove better than when it was in closed loop (TCC lockup demanded in both scenarios).
• When the O2 is unplugged and in open loop, the ECM does not go into into 4th like it does when the O2 is plugged in
• I have logs with the CTS is unplugged but I didn’t look at them yet (attached)

Vehicle info

• 1989 4WD K5
• stock SBC 350 (replaced manifolds w/headers)
• 33" BFG KO2 (stock tires were 28")
• stock 3.41 or 3.42 gears (stamping on front & rear ring gears is 1235128 12 42 9 88)
• Around a year ago I rebuilt the TBI (soaked it in carb cleaner)
• Around a year ago I replaced the
  • thermostat for the ECM
  • injectors
  • IAC
  • MAP
  • ESC
  • Knock sensor
  • EGR valve & gasket
  • bunch of crispy vacuum lines
  • Fuel pump (AC Delco/Delphi FE0115, 12-PSI max)
  • fuel filter (K&N)
  • converted it to an electric fan with a Delta Current Control 70 amp controller
  • Had a local shop rebuild the 700R4
  • Rebuilt the NP241 transfer case

After that work it ran awesome and we took it on a family trip from FL to PA and back with only one issue (had to replace the Cardone steering gearbox with a Borgenson), averaging ~15MPG on 87 octane.

This year before the issues started I

• installed a trans cooler after the rad
• replaced the trans return line with braided AN-6
• added a tee fitting on the remaining hard line for a trans temp sender
• Replaced the upper & lower rad hoses and burped it

I appreciate you reading this far, thanks.

 

[Blue85]

• The ECM is *pulling* fuel the entire time its lugging/surging
• When the O2 is unplugged and in open loop, the ECM does not go into into 4th like it does when the O2 is plugged in

The ECM is pulling fuel because your O2 sensor is reading rich. The sensor is reading rich because the engine is misfiring, which leaves extra oxygen in the exhaust. So this tells you something is wrong, but not whether it's spark, fuel, vacuum leak, whatever.

In a stock 1989 configuration, the ECM has no control over shifting. If it shifts wrong, that's an issue with the TV cable, throttle linkage or things like that. The ECM is only controlling lockup. I'm not sure if there is a pressure switch to tell the ECM when it's in 4th gear. You basically have 2 inputs to control the transmission - the throttle valve and the governor (i.e. vehicle speed).

Why would it ever be in 4th gear at 800RPM with any load? Of course it will lug. It should drop to 3rd. Putting bigger tires with stock gears just makes the lugging worse, but it should essentially raise the shift points, in terms of MPH (which is the same RPM). OD shouldn't come on until 45 or 50MPH, again, depending on your exact setup and the load on the engine. When did you last adjust the T/V cable?

It's interesting to think about the effects of a restricted cooler line, but I think it should still run OK, at least until it gets hot.

 

[clutch2k3]

When did you last adjust the T/V cable?

I haven't (intentionally) touched the TV cable. I'm going to look into this, thanks.

 

[clutch2k3]

I just verified that the TV cable isn't loose. no slack when the the throttle's closed and its tight at WOT (I just added this as a trouble shooting step in the original post).

Just to note, the bigger tires were installed before the 2600 mile trip last year and it didn't lug at all during that trip. It only started lugging after I performed the work this summer.

I didn't mention this in the original post because it was already too long but, about 2 weeks ago while troubleshooting the lugging issue the trans lost all pressure. I had it towed to the trans shop that rebuilt it last year and they said that "a snap ring fell out" and the PR valve dropped into the pan. They put the PR valve back in, changed the fluid and the filter but the lugging issue persists, just as bad as before the most recent rtans work.

I believe that problems with the PR valve could cause it to shift into 4th too soon but could it also explain the lugging in 3rd? Seems like lugging in 3rd is caused by TCC lockup?

 

[Wes Harden]

Tcc locking third is not a normal thing. I don't believe is was common for GM to lock 3rd in the early 700r4, if at all. Mine doesn't lock 3rd unless I want it to.

 

[Blue85]

The pressure regulator valve is inside the oil pump assembly. It faces downward, so you can put it back together from below with the pan off. Why would it come out? Maybe it was never installed correctly, maybe the snap ring broke.

 

[Blue85]

So why is it shifting so early? With your tire/gear combo, it shouldn't drop into 4th until 50 or 55MPH and you're saying it happens at 35MPH. Is your speedometer correct or are you using GPS? At any rate, that's way too early. Also, how are you measuring RPM? Over ALDL?

• TV cable is loose, or is the wrong cable for the application, the linkage inside the transmission has come loose or the linkage at the throttle body is wrong. If you really want to know if TV is working right, there are pressure checks (below)
• The governor is messed up. The typical failure is a stripped gear, which gives the opposite problem (really high shift points). Maybe the springs have broken?
• A sticking valve in the valve body
• Oil pump problem?

 

[Blue85]

How is the shifting otherwise? Firm or sloppy?

 

[clutch2k3]

@Blue85 & @Wes Harden I'll respond to the questions/concerns in between my work meetings today.

I looked into the "normal" logs I attached to the OP (20250818_194722_LOG_normal-driving.csv) and the shift points look normal to me in that log file (however it was lugging while those logs were captured). Here's a log file I captured a few weeks back thats more typical of how its currently driving. It shows TCC Lockup in 3rd at 32MPH/1800RPM and then a 3-4 shift at 37MPH/1450RPM and TCC Lockup was still demanded (let me know if you want the raw CSV or if I have interpreted the logs incorrectly). Whlie in 4th with TCC Lockup the RPM dropped to 925 (not 800 like I originally stated, I updated the OP) but it stays in 4th with TCC lockup and lugs like crazy.

... Why would it come out? Maybe it was never installed correctly ...

I agree 100%, fully seated snap rings don't fall out. I believe either it wasn't installed correctly or some change I did to the vehicle this summer caused it to break.

... it shouldn't drop into 4th until 50 or 55MPH and you're saying it happens at 35MPH. Is your speedometer correct or are you using GPS? ... how are you measuring RPM? Over ALDL?

My speedometer is not correct. The MPH & RPM values I'm stating are from the ALDL logs.

 

[Blue85]

But the ECM is getting speed from the same place the speedometer is. I think the VSS sender in the transfer case didn't start until '90 or '91, so an 89 would still have the speedometer cable running the speedo and the VSS comes from the speedo. With bigger tires, you're going faster than it thinks.

Clearly, the ECM does have feedback from the 4th gear pressure switch.

Is the fueling really that bad? INT is STFT and should be 128 +/- some small amount. Then it will push BLM, which is LTFT. I don't know TBI enough to know if being down 8 counts is a big deal, but that should be like a 6% change in fueling. But like I said, if it's misfiring from lugging, it might not actually be rich. If it pulls enough fuel, it will do lean misfire and look even richer.

But is it really lugging when TPS is 5%? It's weird for the transmission to shift at 1450RPM, but it's because you lift from a mere 9%. What is the shift point at a normal level of acceleration? If you apply some throttle there at 35MPH, does it jump right back to 3rd gear?

 

[Blue85]

Tcc locking third is not a normal thing. I don't believe is was common for GM to lock 3rd in the early 700r4, if at all. Mine doesn't lock 3rd unless I want it to.

A lot of them did. I had a switch on mine for years that selected between 4th only and 3rd/4th. Saying that yours locks when you want it to, implies that they can.

 

[Wes Harden]

So mine is completely custom. No GM controls or strategies for tcc lock up

 

[Blue85]

It's interesting to think about putting a pressure regulator valve back together wrong. Is it possible? What happens?

 

[Blue85]

For the sake of troubleshooting, you can keep the converter unlocked by applying the brake pedal just enough to trigger the switch. You can also just unplug the transmission, but driving much in OD with the converter unlocked produces a lot of heat and running the pump <1000RPM doesn't flow much oil to the cooler.

 

[clutch2k3]

@Blue85

... an 89 would still have the speedometer cable running the speedo and the VSS comes from the speedo.

Good to know! I kinda figured this by looking at the speed values in the logs but you just 100% verified this, thanks

(is) being down 8 counts is a big deal ...

From what I've read 8 counts for a stock engine is a bit far

... But like I said, if it's misfiring from lugging, it might not actually be rich. If it pulls enough fuel, it will do lean misfire and look even richer ...

I never thought about how a lean misfire will show up as a rich condition, it's so counterintuitive, again thanks. What a horrible cycle the ECM/O2/INT/BLM are in! This actually supports my working theory that this condition was always present but the bad O2 masked it. I also noticed with the O2 unplugged it wasnt shifting into 4th gear. maybe becuase the gas pedal is in a different postion changing where the TV cable/valve are at? While in open loop without a good O2 the oblivious ECM would just keep feeding it fuel, but now that it sees a lean misfire from lugging the silly computer makes it worse.

For the sake of troubleshooting, you can keep the converter unlocked by applying the brake pedal just enough to trigger the switch ...

I installed a TCC lockup override switch. I'd guesstimate 90% of the lugging issue goes away with TCClockup orverridden.

 

[Blue85]

I never thought about how a lean misfire will show up as a rich condition, it's so counterintuitive, again thanks. What a horrible cycle the ECM/O2/INT/BLM are in! This actually supports my working theory that this condition was always present but the bad O2 masked it. I also noticed with the O2 unplugged it wasnt shifting into 4th gear. maybe becuase the gas pedal is in a different postion changing where the TV cable/valve are at? While in open loop without a good O2 the oblivious ECM would just keep feeding it fuel, but now that it sees a lean misfire from lugging the silly computer makes it worse.

Sometimes sensors lie, so you need the right frame of reference. It's easy to think these O2's are telling you AFR, but they are not. They function like a switch which tells you the Oxygen is above or below the expected amount. The only way this really translates to AFR is when it's rapidly switching. If you are not at 14.7:1 (or whatever lamba your fuel uses), there is really no information.

Something that just came to mind is that there are apparently some sensors that depend on the wire to bring in a trace amount of fresh air. If you splice the wire, the sensor doesn't work right. I don't know how to look that up for any particular part number.

How much higher is the RPM when it's unlocked? It's possible you just move the engine up into a region it's tuned better for. There could still be some other problem like an exhaust leak, vacuum leak, stuck EGR, etc that has the most impact when the airflow is lowest.

I still don't think it should be running 1000RPM going down the road. If it's lugging you'll be forced to apply more throttle and it should downshift. Any chance you ended up with a diesel torque converter or a diesel governor?

 

[clutch2k3]

But is it really lugging when TPS is 5%?

Yes, its lugging like crazy when TCC is locked even with light throttle.

What is the shift point at a normal level of acceleration? If you apply some throttle there at 35MPH, does it jump right back to 3rd gear?

IDK. I'll test and get back

Sometimes sensors lie, so you need the right frame of reference. It's easy to think these O2's are telling you AFR, but they are not. They function like a switch which tells you the Oxygen is above or below the expected amount. The only way this really translates to AFR is when it's rapidly switching. If you are not at 14.7:1 (or whatever lamba your fuel uses), there is really no information.

I never thought of it this way, makes so much sense.

.. there are apparently some sensors that depend on the wire to bring in a trace amount of fresh air. If you splice the wire, the sensor doesn't work right.

mind ... BLOWN. My BOSCH 12014 narrow band is in fact one of them. I wrapped it's connector in heat shielding. I only wanted to protect the extra wire that was hanging but threw the connector in there because I didn't know any better.

How much higher is the RPM when it's unlocked?

From what I remember a few hundred RPM. I'll retest and get back.

It's possible you just move the engine up into a region it's tuned better for. There could still be some other problem like an exhaust leak, vacuum leak, stuck EGR, etc that has the most impact when the airflow is lowest.

I agree (see bottom) but exhaust leaks, vacuum leaks and stuck EGR valves are tomorrow problems :-)

Any chance you ended up with a diesel torque converter or a diesel governor?

If I did, it worked fine from FL to PA and back.

Assuming the problem started when the 33's were installed but was masked by the bad O2, I might be able to:
- Clear the BLM table
- Repopulate the BLM table with TCC lockup overridden
- Only allow TCC lockup under the right conditions with 33's + stock gears

 

[clutch2k3]

I no longer believe the problem started when the 33's were installed but was masked by a bad O2, I now think the torque converter is bad. The torque converter just started making the same “tin foil scraping” sound like it did last year before it completely went bad and had to be towed. I'm not sure if some other problem messed up the torque converter or if a bad torque converter is the source of the issues. I attached a video with the sound of what I believe is a bad torque converter.

I cleared the BLM table, drove ~15 minutes with the TCC override switch engaged (preventing the ECM from locking up the TC), got it up to 65MPH (GPS), was using only light throttle to maintain speed and then let the ECM lock up the TCC and it still bucked pretty hard. I don’t think its possible to lug in this scenario and this is exactly when I want the TCC locked up. The bucking is so bad its vibrating the truck and you can hear the interior of the truck vibrating.

- Before the test drive I took the O2 connector out of the heat wrap
- It shifts somewhere between firm & mushy depending on the scenario. While going ~30MPH I downshifted into 2nd and it felt really firm. While going ~40MPH I shifted between 3 and 4 a few times and it felt firm enough. But this is subjective and I don’t think I have enough experience with different K5s to be a good judge of this.
- I wasn't able to see the exact shift points during normal acceleration because the laptop is off to the side but wherever its shifting seems fine to me. If these values are important I’ll dig them out of the logs by looking for higher TPS with RPM dropping and getting the corresponding speeds.
- When I applied a decent amount of throttle at 35MPH it goes back to 3rd gear
- TCC lockup lowers the RPM between 200-600 RPM depending on the scenario. On side roads, ~200RPM; on the highway, ~600RPM. I could get more accurate values from the logs by looking where TCC remains locked but there are drastic RPM changes, that’s when I was flipping the override switch.
- When its bucking and I apply a good amount of throttle it goes out of TCC lockup and downshifts

I attached all the log files from tonight's test drive.

View attachment 2025-08-28 23.34.00 (1).mp4

 

[Blue85]

So you think the torque converter clutch is grabbing/slipping or are the misfires just more noticeable under those conditions?

 

[clutch2k3]

I took it back to the transmission shop and they said nothing's wrong with the TC. I'm thinking I confused the sound of a bad TC with an exhaust leak that's only audible during low idle.

I'm back to thinking the idle and cruise problems are due to a vacuum leak. I haven't had time to try and find it though.