3g stumble

Some thoughts:

When you say timing is spot on, do you mean mechanical timing, ignition timing, or both?

Is the new distributor a Toyota OEM distributor or aftermarket? Same question for the igniter and coil.

The car has many "VSV's" - which VSV is new?

Why did you remove the charcoal canister? And what does "vented to air" mean?

Is your check engine light on now? Or does it come on when you engine gets to 3,000 RPM and starts to stumble? Have you checked for any DTC's (even if the check engine light is off when the engine is running)? Does your check engine light otherwise work properly (is on when engine is off and key is in ON position; is on when engine is started and for a few moments after engine starts before it turns off)?

There are some diagnostics that can be performed on that OBD-1 ECU, but before going there, just would like to work on some answers to basic questions first. I also have a Celica convertible, a 1993 model.

Kinda got your '93 rag top from your moniker
The check engine light works great, comes on with the key turned on and off after starting. the engine runs great, shifts on time.
There's only one vacuum switch on mine. See photo.
The cel does not come on when it stumbles. Almost feels like it's not getting any gas but that's not the case.
Fuel pressure is right up there.
Removed the charcoal canister off because it was rusted on the bottom. Vented to air means the line to the gas tank return line is now open. the temp sensor that connects to the canister is left open also. A rebuilt ECU was installed two years ago.
Doing the OBD test reveals everything is normal.
Think the canister might have anything to do with it?? I can blow it out, patch it and re-install just to satisfy my own curiosity.
Timing is mechanically correct with the belt and camshaft. Not looking at the specs right now but I think I have it timed at 8 deg Btdc.
Opinins???
it's my summer driver which is ok. I'm 78yrs old, don't beat the s... out of it. Bought as a project car in '21. Replaced the trans last year .

I'm assuming your 91 Celica GT has air conditioning. If so, you have 3 VSV's (circled in the same picture you provided below).

Does your engine exceed the 3,000 RPM limit when the engine is started cold and is rev'd up immediately? If you can get it above 3,000 RPM while the engine is cold, can you get it up to 6,000 RPM? Is the 3,000 RPM limit only happening after the engine is fully warmed up? Did the engine not have this low 3,000 RPM limit immediately after you removed the charcoal canister from service, or did the 3,000 RPM limit happen immediately after the removal of the charcoal canister?

Your ignition timing should be 10 deg BTDC with the engine in diagnostic mode. Is this how you set your ignition timing (while the engine was in diagnostic mode)?

Also, why is the title of your message 3G stumble?

You did not answer the question from my previous post regarding your distributor. Please also answer that question.

Knowing the answers to these questions will help shape my opinion back to you.

I think if your ignition timing is now 8 deg BTDC w/o being in diagnostic mode, your ignition timing is severely retarded. So that could be your primary issue. Since your stumble is 2,000 RPM when the engine is cold and 3,000 RPM when fully warm, I don't think the charcoal canister modification you made is the primary issue, but I do think it is a secondary issue.

If you have air conditioning, you have an idle-up VSV on that engine. That VSV is circled in red on the photo below of a 1991 Celica GT. The vacuum lines run to a regulator mounted on the passenger side of your intake manifold. My 1993 Celica uses a totally different air conditioning idle-up VSV, but the principle is the same as yours. It is dim on your vacuum hose information sticker because it shows it as being needed for units made with air conditioning - which yours has.

Once you have your ignition timing properly set, see what you have for engine RPM capability. Then we'll talk more about the charcoal canister. I don't think it wise to have it removed - for sure not wise to have it disconnected in the way I'm interpreting from your responses that you have it disconnected now. But that can come later.

I will be traveling for 10 days after tomorrow - so won't be able to respond back during that time - just an FYI.

I did a re-time in diagnostic mode, it was off slightly.

Got lucky on this pic, the engine was running and hit the timing mark dead on.
Took it for a run and the issue is still there. I did take a video but have to learn how to edit it. Later...
It threw a code 14 during the "stumble" but then went away after I slowed down.
Just might have something to do with that evap.

Here is what code 14 means and the troubleshooting pages (FI-109 for general information & FI-119 for this specific DTC 14) are attached.

IGT is the ECU signal to the igniter and controls ignition timing. IGF is a signal from igniter to ECU and tells the ECU if the coil actually fired.

Given you don't have the original distributor (not sure if your current distributor is OEM or aftermarket), and you ECU was repaired at some time, the problem could be with either of those. Could be a wiring problem also. Not sure you want to check things out that are in the repair manual for this DTC 14 or not (hard to get to the ECU and the wiring harnesses to check voltages and all). But that appears what needs to be done at this point.

Yup. I have the manuals and was just checking them came up with the same info you gave. I did replace the igniter with an aftermarket. I'll put the original back in and check all the wiring. Also putting the canister back in just for giggles, won't hurt.
If this was a 50's or 60's Ford or Mopar I could do this with my eyes closed. First time I've ever worked with ECUs, igniter, dizzy with coils etc. I'm learning.....I think. Thanks again for the help.

You're right about aftermarkets.
The distributor is OEM but remanufactured. had to order it out of Huston, TX. through NAPA auto. $365. 0uch, that one hurt and I had to exchange the core old one.
Put the original igniter back in, started right up. The aftermarket one had a little difference in the female connector, just didn't seem right now that I took it off. There's the usual "This will fit your car" claim when looking at parts. They fit ok but they are not the same. Lessons are always learned better at a price.
Will be putting the evap in tomorrow. Just got it all cleaned up and painted to look purty.

Chiming in late on the stumble:

- 93 is right to check the ignition items. There are known issues with capacitors from that generation as well as problems with heat/vibration causing problems with old wiring and electronics.

Other:

• make sure you are running on all 4 cylinders. Verify spark signal on each. You might check each plug and holding it near the block while cranking to verify none are broken. Use a mechanics stethoscope to verify that each injector is ticking.
• the cold injector could be leaking causing a rich environment. Pull it, turn fuel puMp on and verify no leakage.
• verify the engine goes into closed loop (and thus not running rich). It should idle up on cold start and then drop 300-400 rpms going into closed loop.
• the ecu is programmed with a std air/fuel table that it uses to set injector rate based on sensors. The tps sensor gives feedback on what you are dOing with the accelerator and the map tells how much air is flowing (vacuum amount). The ecu sets initial injector signal with these sensor readings and then adjusts based on o2 signals. The service manual gives procedures on how to check all of these sensors. You should also consider repeating each test by taking readings at the ecu to account for possible wiring problems. You could be getting away with slightly off readings at low rpms but be starved at higher rpms.
• if you have a vacuum leak, then the engine is likely in a idle hunt when warm. This is exemplified by rising and falling off the idle rpms.
• EGR is supposed to open at mid rpms and close at high rpms. Your egr might be stuck open.

Sent from my Pixel 5 using Tapatalk