^^ I second this.

If the PCM is working properly then it will show you, with the OBD1 code flashing system, if any error codes are registered. Also if the PCM is working properly and there are *no* error codes present you it will, if put into the OBD1 code retrieval mode, do a simple rapid flash of the CEL light which will confirm for you that there are no active error codes present.

If the little light bulb behind/on the dash cluster that illuminates the CEL light has gone out... then you won't know anything about no error codes or even one error code until the bulb is replaced. So if that light illuminates under any circumstances you can rule that out.

If the PCM actually has got an internal issue (almost always bad capacitors) and thus requires professional repair then you *may* have some irregularities getting it to show you anything consistent with the CEL light at all... in addition to running issues.

...

As t2d2 said, you need to get a different and more astute mechanic to take your SC to. OBD1 diagnostics is very simple to use and there is a very straightforward step-by-step ruling out procedure in the SC300/400 TSRM (factory Lexus service manual-- available online in PDF form) to determine whether or not the PCM or some other thing is causing the irregularity if there is any question and hesitation that it is the PCM that is actually causing the issue or not.

 

Ok thanks. Yes I am having doubts about the mechanic. This is what the mechanic sent to me today regarding a diagnosis. They said they don't think it's the PCM despite the trouble codes not displaying and do not seem super inclined to swap out the PCM given that they do not think it's the issue and the extra labor, etc. The CEL does work, intermittently.

Technician observed that check engine light is not illuminated or flashing at time of inspection. Vehicle will intermittently drop RPM and stall; attempted to retrieve diagnostic trouble codes and check engine light flashes continuously with terminals E1 and TE1 connected with a jumper lead. Service information provides no protocol for continuously flashing light when attempting to retrieve DTCs/unknown if this means no trouble codes are stored or if an outlying issue is present. Additionally technician observed that when going into "Test Mode" diagnostic trouble code retrieval procedure, check engine light does not flash key on/engine off indicating a problem with TE2 circuit which is integral to ECM.

Technician recommends a baseline of 2 hours in total for proceeding with diagnostics due to intermittent concern on pre-OBD2 vehicle, all components and circuitry must be manually tested. Vehicle is currently in a state of partial disassembly.

 

I also have to admit that various lights on the dash need to be replaced as they're out.

 

To clarify ... the CEL will illuminate at Key ON, so you can verify that the bulb is functional. No illumination at that stage, replace the bulb before doing anything else.

spdim, from the last two posts, you obviously have a functional bulb. I'm not sure what to make of "check engine light is not illuminated or flashing at time of inspection" and "check engine light flashes continuously." That seems contradictory. The latter would suggest that the PCM is okay and that the problem lies elsewhere. Maybe the first quote just means there's no stored CEL keeping it lit up outside of test mode?

But ... at the start of the thread you had a CEL lit up, so I'm confused.

 

Thanks for that. Before I brought it into the mechanic, the CEL would flash and then go off. But I obviously don't see what the light is doing when they're testing the car so I'm a little confused too.

 

spdim, remember that with an OBD1 pre-1996 vehicle the Check Engine Light will only stay illuminated if the error or problem remains constant.

OBD1 diagnostic systems do not keep codes stored for long-- say if your CEL comes on and off while you're driving down the road and then stays off by the time you can pull over to check out the issue then you would have to wait until the CEL comes on again before you would be able to put the car into diagnostic mode to check which error codes come up.

If a CEL comes on while driving and then goes off again in an OBD1 vehicle it means the issue is intermittent and the PCM went through enough cycles to clear it out again.

With a 1996+ OBD2 vehicle these momentary error code events are stored and have to be cleared out manually with a scanner... but that does mean it makes it easier to see what codes were triggered while the engine has been on if the error has been intermittent/transient.

But with OBD1's if the error code isn't flat out constant you have to sort of "catch it in the act" before the PCM goes through enough cycles to forget about the last instance... until that error code CEL is triggered to appear once again.

I am also confused by the statements your mechanic made to you. And t2d2 is right-- you seem to have a working CEL bulb even if you say some of the other cluster bulbs need replacing (a fairly easy job to do yourself after ordering a new set from Toyota or Lexus).

 

Kahn nailed it regarding catching it in the act. When you have the car back, it would be worth taking instructions with you on retrieving the codes so you can look them up as soon as the CEL comes on. There's probably a correlation between the light and the car's bad symptoms...

 

Thanks guys. It's frustrating because I'm not sure how much confidence I have in the mechanics who have already done their testing for 4.5 hours and will now charge for additional testing and I already paid them 850 bucks to replace the pcm in June and am now wondering whether to get rid of the car at this point.

 

From the sound of the issue(s) so far I don't think the car has anything catastrophically wrong with it. Even if you needed another ECU they're still fairly easy to come by in the correct part number if you have an automatic SC.

I think this is just an unfortunate and certainly frustrating case of auto mechanics who are used to working on much newer cars and who aren't nearly as familiar as they should be with older OBD1 cars.... which really aren't all that different than later OBD2 model years.

Individually testing certain components to rule everything out but the true culprit is a valid method of automotive electrical repair... but with anything made after 1985 or so you do it with the help of the engine computer's diagnostic mode, however simple and old it may be by today's standards. Most good repair shops have AllData which should give them access to ALL the Lexus SC300 TSRM manual pages and the SC300 wiring TSRM if they needed it. Every procedure we've discussed in this thread is clearly laid out in the factory TSRM. We've just been discussing the cliffs notes version for the OBD1 diagnostic port with you to speed your process along.

I wouldn't give up. These really are not complicated vehicles to diagnose compared to the much more modern vehicles that your repair shop probably sees and diagnoses with much more advanced service computers. They most likely are not very familiar with these cars or older Toyotas in general.

...

Other than a regular ignition tuneup and having clean and balanced injectors (ie: sending injectors to a place like Driftmotion, RC Engineering or Five-O Motorsports to be cleaned and balanced professionally and sent back to you) I would guess without having any evidence that the issue might be related to any of:

--ECU issue
--failing igniter (very rare)
--failed, failing or damaged hot-wire intake air sensor (which should NEVER be cleaned by the way!)
--super severely clogged EGR system or a malfunctioning EGR valve (but I don't think a clogged EGR valve would cause the engine to shut off)
--a bad, frayed or broken wire somewhere that is causing an open short or incomplete circuit (inspect connectors in the engine bay visually for this)
--a failed sensor, such as the ECU coolant temp sensor, something in the Idle Air Control Valve, etc., etc. (other than a bad wire a CEL error code determines a sensor failure usually)

However without seeing the car, noting its symptoms, running some tests and checking the OBD1 diagnostic code retrieval system properly when the code shows up I have *zero* evidence (yet) to stand by any of those above possibilities.

 

Kahnbb6, thanks very much for the words. I'm taking it to another mechanic tomorrow. I obviously do not know about this stuff. My friend who is a mechanical engineer explained to me how engines work and this all makes sense now. I hope this guy my friend recommended can get to the bottom of it. Thanks again for the encouragement as this has been expensive and frustrating and all my friends are telling me to get rid of the car.

 

I completely understand and I sympathize as I have been in that situation before with everyone around me locally telling me to get rid of a problematic car (when I still had a fair idea of how it could be fixed once the true issue is discovered). I have found that the secret is really just to methodically rule out each possibility through testing and going through the logical ascending order of testing/replacing one at a time as outlined in the Lexus factory technical service manual.

But sometimes we who have owned these models for a long time generally know what the usual suspects culprits may be and can narrow down the investigation just a bit.

As older cars go this is still a very over engineered model (and a special model in the Toyota/Lexus history IMO) that is worth getting fixed if, when it's issues are finally behind you, it still brings you joy to drive and own.

Your engineer friend is good to help you out to explain the fundamentals and to get you in touch with a much better mechanic I think you will have better luck with this once you've got it in the care of a much better mechanic who knows his stuff

If you have further issues that come up while your new mechanic helps diagnose the issues or if you need any recommendations on where to buy or find parts as necessary check back in with us and we'll do our best.

Hopefully this will be the worst of it and your SC will be made whole again soon

 

Thanks so much. I really appreciate the kind words. It does bring me joy to drive so I hope this guy can do the trick. As you said, I hope it's something small. Seems to be a needle in the haystack with the issue.

I will reach out when I know something.

 

As an update, a more competent mechanic than the last guy believes it is the air intake control valve and is ordering one. I'll keep my fingers crossed.

 

That could be possible that it's the IAC-V (Idle Air Control Valve / Module). Don't rule out the sensor portion of the Varmann-Vortex sensor (aka older style MAF sensor) that bolts to the air box. Remember this rule: NEVER CLEAN THAT SPECIFIC SENSOR (your MAF sensor). If that part is ever in question there should be some replacement used OEM units available on ebay and some available as new aftermarket versions on RockAuto.

See what happens with the new Idle Air Control Valve first.

 

I don't recall whether the SC300 IACV is the same setup as the SC400 one, but on the latter, the connector itself can be the problem if it turns out to be an IACV issue. The connector angles down and doesn't maintain good contact as gravity works against the old plastic. I fixed a problem simply by zip tying the connector tight against the valve, before eventually replacing the connector.