I have been reading every 3FE code 25 and 26 thread on this site, and after 4 weeks of troubleshooting I'm bringing this one to the community.
Background: (TLDR: total engine rebuild after overheating, getting code 25/26 after ~30 mins of driving around neighborhood)
In the summer of 2022 I took our LC from San Diego to Yellowstone, with a meandering trip through the lower Sierras on the way back. In the mountains it handled fine, but once in the central valley and through the Grapevine in +100 degree heat, the temp needle finally began to move north. We managed the heat as best we could, and made it back without her dying on us. I thought that was pretty bad ass. After getting back it turns out the money I poured into cooling system repairs were foiled by the shop failing to replace the fan clutch we agreed to, and I failed to confirm it was replaced. Lesson learned. After sitting for a couple months I took it out into the Cuyamaca hills and noticed a pretty big drop in overall power.
That led to me to a compression test on two cylinders, which led to taking the cylinder head in for a valve job and resurfacing, which led to the discovering the block was warped, which led to an R&R and engine rebuild. What can i say, I'm not giving up on the 3FE. Now everything is reinstalled, and my new slight overboard (0.003") pistons are matched with my previously working Beck/Arnley 158-0580 injectors.
First time experiencing code 25/26
The shop doing the rebuild called to let me know that though it seems to be driving fine, the Bruiser was throwing 25/26 codes after ~30 mins on the freeway. I had a new pair of O2 sensors (Denso Oxygen Sensor 234-4150 and 234-4051), so we replace the originals with the Densos. The errors persisted. At this point the shop said they called in someone who knew more about 3FEs, and diagnosed the ECU wasn't "getting signal from the O2 sensors". I took the car back as it was in their shop for almost 5 months, and got to work on troubleshooting.
Work done prior to R&R:
At this point I'm looking at that FSM prescribed O2 Sensor test starting at FI-75 and am considering a new ECU. The only remaining things I can think of are to verify the valve clearances (valve clearance was set prior to engine replacement, so cold), inspect the distributor more closely, and test the dashpot (because of the dip in RPM between Park and Drive). Any thoughts from the community on what I could check before going for the new ECU?
Background: (TLDR: total engine rebuild after overheating, getting code 25/26 after ~30 mins of driving around neighborhood)
In the summer of 2022 I took our LC from San Diego to Yellowstone, with a meandering trip through the lower Sierras on the way back. In the mountains it handled fine, but once in the central valley and through the Grapevine in +100 degree heat, the temp needle finally began to move north. We managed the heat as best we could, and made it back without her dying on us. I thought that was pretty bad ass. After getting back it turns out the money I poured into cooling system repairs were foiled by the shop failing to replace the fan clutch we agreed to, and I failed to confirm it was replaced. Lesson learned. After sitting for a couple months I took it out into the Cuyamaca hills and noticed a pretty big drop in overall power.
That led to me to a compression test on two cylinders, which led to taking the cylinder head in for a valve job and resurfacing, which led to the discovering the block was warped, which led to an R&R and engine rebuild. What can i say, I'm not giving up on the 3FE. Now everything is reinstalled, and my new slight overboard (0.003") pistons are matched with my previously working Beck/Arnley 158-0580 injectors.
First time experiencing code 25/26
The shop doing the rebuild called to let me know that though it seems to be driving fine, the Bruiser was throwing 25/26 codes after ~30 mins on the freeway. I had a new pair of O2 sensors (Denso Oxygen Sensor 234-4150 and 234-4051), so we replace the originals with the Densos. The errors persisted. At this point the shop said they called in someone who knew more about 3FEs, and diagnosed the ECU wasn't "getting signal from the O2 sensors". I took the car back as it was in their shop for almost 5 months, and got to work on troubleshooting.
Work done prior to R&R:
- New TPS - bench tested and adjusted per the FSM.
- Cleaned ICV and applied new gasket and screws.
- New PCV valve and grommet.
- New Brake Booster.
- New Master Cylinder will full bleed (all 5 points).
- All new gaskets for valve cover, intake/exhaust assemblies
- Fan clutch replaced.
- Thermostat replaced
- All new vacuum tubing.
- New FPR (GP Sorensen 800-200) and fuel dampener
- New fuel filter
- New O rings and gaskets for the injector cups and injectors.
- New sensors on thermostat housing (all OEM)
- The left cat was replaced 4 years ago with an aftermarket weld on one, and the right is OEM. Left had loose medium rattling around inside. This combo worked all the way until the R&R with no codes thrown and passed smog twice.
- Vacuum leaks
- I've checked and rechecked every hose. I tried both starter fluid and propane gassing to find anything new. No luck. I replaced all the smaller hosing last year. The boot between MAF And throttle body are new. All hard lines were cleaned and cleared with air.
- Vacuum meter attached to the manifold at the splitter that goes to the BVSV shows ~18.5mmHg, but it's not steady. The needle would vibrate +/-1mmHg constantly at idle (warm and cold), and today I noticed sharp momentary dips down to 15-16mmHg. Today was the first time I've seen this larger jump, the rest of the last 4 weeks have seen only +/-1mmHg vibrations. I did reset the ECU with the 15A EFI breaker pull method prior to this test.
- Timing
- Timing was definitely off when I got my first look. I'm not sure how it was running, but timing was set with TDC was in view when a simple trigger timing light was attached to cylinder 1 (with jumper in E1 and TE1 of check connector). I advanced to the 7BTDC mark, and idle felt a bit better, but vacuum was vibrating just at 17mmHg, so I advanced until I got around 18.5mmHg. I'm guessing this is around 10BTDC, validated with an advanced timing light. Setting it to 12BTDC smelled too rich.
- Idle screw is about 3/4 turn from tight - this was set after the above timing effort, and idle sits at 650.
- (SIDE NOTE) I might be using the Innova 5568 timing light incorrectly, but I noticed that when connected to cyl#1 my advance is closer to ZERO than TDC. I originally set timing using a cheap trigger pull timer, and used the Innova to help with advancing. When I set the timing to say +10 on the Innova, I can only see the TDC marks when adding +7 degrees. Any tips here to understand this would be helpful.
- EGR Valve
- EGR valve itself checks out - I applied vacuum to port facing the manifold and engine instantly stumbled. It regained idle after releasing vacuum.
- Exhaust leaks
- Smoke testing didn't really help with this one, but i could smell the smoke around the driver side tire, so I tried the soapy water test. This yielded two leaks at the newly installed O2 sensors. I fixed one with some tightening, and mostly closed the other with tightening but I do still get a little bubbling. I plan to address this next weekend.
- No other exhaust leaks were found at the manifold or the manifold-to-downpipe connections.
- O2 cable integrity checks
- Using my Innova mulitmeter, I checked each cable coming from the O2 sensor connectors to the ECU. All cables have continuity, and resistance on each was below 1.5ohms.
- FSM O2 sensor test at Check Connector
- Using the troubleshooting sequence in the FSM:
- E1 jumpered to TE1 and a multimeter with GND in E1 and POS in VF1, idle reads ~0.65V. Holding revs at 2500rpm for 10 seconds, the multimeter holds at 5V. I see no dips in voltage.
- Same test for VF2. Same results. Idle measures ~0.65V, and 2500rpm measures 5V continuously with no dips.
- Per the FSM this test is a failure and suggests replacing the ECU.
- Using the troubleshooting sequence in the FSM:
- FPR validation
- Fuel pressure at startup varies, but is usually around 20-27PSI. It raises to around 32psi when it gets warm.
- FPR holds around 20psi after 5 minutes of engine OFF.
- Disconnecting the vacuum line brings it up to 40+psi, and reconnecting the vacuum line brings it back down to the low 30s.
- ECU inspection
- Outside: All connector pins look in good shape. No burn marks or corrosion
- Inside: all components inside appear to be in good working order. Nothing damaged, all solder joints are clean, all caps are normal size (not budging), all voltage converters look healthy, and the ICs appear good (no bubbling, melting, or other damage), and nothing is loose.
- Remaining symptoms
- When warmed up, going from park to Drive sometimes dips the RPM momentarily >600rpm, but regains to 650 and holds.
- Not a terribly smooth idle. It's not terrible, but I can feel very slight stumbling.
- I can hear a slight popping sound from the exhaust. It sounds deeper in the system than just at the tail pipe.
- Very small exhaust leak at the passenger side O2 sensor bung.
- Brakes occasionally go spongy. Quick recovers on next brake push.
- Error code 25/26 after driving around my neighborhood for 30 minutes. Lots of chances to rev 2500+rpm climbing hills for 15 seconds, engine off and on to test Cold Start Injector, etc.
At this point I'm looking at that FSM prescribed O2 Sensor test starting at FI-75 and am considering a new ECU. The only remaining things I can think of are to verify the valve clearances (valve clearance was set prior to engine replacement, so cold), inspect the distributor more closely, and test the dashpot (because of the dip in RPM between Park and Drive). Any thoughts from the community on what I could check before going for the new ECU?
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