fkucf10

I'm struggling to diagnose/remedy an issue with my '93 UCF10. My harmonic damper recently failed catastrophically (ate the no. 1 timing cover and crank position sensor) so I have just 'completed' a timing belt job (replaced the crank sensor, timing belt, no. 1 timing cover, distributor caps). Upon reassembly the car fired up instantly, but there is an immediately noticeable harmonic at idle probably best described as a beat phenomenon. This is noticeable in both engine RPM and what I would classify as a mild to moderate chassis vibration. Idle RPM is about the same as before the repair. Upon applying some throttle the tachometer drops to 0 at ~1000rpm. The engine revs normally and smooths out above 1000rpm, but bogs on return to idle if the reduction of throttle is at all abrupt. No trouble codes are thrown, even after 10+ minutes of running with multiple revs/holds at higher rpm (~3000 based on engine note).

I have taken the following troubleshooting steps:

1. Replaced the No. 2 igniter engine harness connector as it was quite brittle after 30 years in the engine bay
2. Removed the engine harness connection from the No. 1 coil (engine fires but immediately dies, trouble code 14)
3. Removed the distributor connection from the No. 2 coil (no start condition, just wanted to confirm the No. 2 coil was working)

I would appreciate any help/ideas the forum has to offer, because this one has me stumped. My research points me towards something with the No. 1 (Left bank) igniter since that's what drives the RPM signal. I am in the process of confirming my LH distributor rotor installation, and would be open to the possibility that I cocked up the LH igniter somehow, but the lack of trouble codes and the acute onset of the problem have me questioning a component failure.

fkucf10

Could anyone measure a new distributor rotor contact? I see some strange wear on both the rotor and cap contacts from my LH distributor.

deanshark

First off, sorry to hear about your troubles with the harmonic balancer, I was just talking about this failure about a month ago. I changed mine last year so this failure wouldn't happen to me. As soon as I read your first post I instantly thought of the rotor being not on correctly. (180* off) Lots of people have done it, by accident of course. If the rotor and cap now have the strange wear on them you should get some new ones anyway. Did you put new ones on when you did the T-belt job?

fkucf10

Dean, thanks for the reply. The rotor being off 180 degrees was one of my first thoughts as well. (Un)fortunately I was very careful to make sure the key and keyway were aligned during assembly, and I confirmed they were still aligned when I removed the rotor this afternoon, so I think that can be ruled out. I'll set the engine to TDC and confirm that both were correct when I get to the RH rotor in the morning. I installed new rotors, but not new caps, as part of the repair. I measured the contact on the rotor I removed to be less worn than the brand new rotor which has run only long enough to bleed the coolant system and diagnose this problem.

I think my next step will be to pull the RH rotor off to compare wear patterns. Given how smooth the engine is above idle I suspect the issue to be limited to one distributor, so hopefully there will be something to learn there. Worst case I've got an order queued up with mylparts to replace the ignition system (coils, plug wire kit, coil harness connectors, rotors, caps).

Yamae

I worry that the crank position sensor is not generating enough voltage signal to drive the ECU's timing circuit. I mean that the amplitude is not enough. The crank position sensor's output voltage is rpm dependent. It saturates as rpm rises but under 1000 rpm, the amplitude is smaller compared with higher range of rpm. There are two reasons that make the amplitude low or small. One is the distance between the sensor and the rotating tooth. The other is the sensor itself. Many aftermarket sensor does not generate enough voltage, I was told. If I were you, I'd check the waveform of it first. The amplitude should be like this (lower part)

fkucf10

Thank you for taking the time to help, Yamae. I cannot rule that out as I don't have an oscilloscope on hand, but the sensor is a brand new Lexus part from mylparts.com. I will confirm the orientation of the trigger wheel later today, but I was careful to make sure the trigger wheel was installed correctly.

I was under the impression from this post that the tachometer signal was driven by the No. 1 igniter. I wasn't able to find a similar diagram explicitly for the UCF-10, so could you shine some light on whether the No. 1 igniter is the source of the tachometer signal? I know the ECU receives signals from both cam position sensors, the crank sensor, and both igniters, but the crank and cam sensors should throw trouble codes so the igniters seem to be the most likely culprits.

fkucf10

I have confirmed that the engine is in time (according to cam and crank pulley markings). Both distributor caps are similarly worn and their keyways correctly aligned (though I'm still puzzled by the new cap contacts being thinner than the ones I pulled out). The crank trigger wheel is correctly aligned (and, therefore, on the keyway, as had it somehow come off the keyway during crank pulley install it would be deformed by the crank pulley install torque or too far forward on the crank nose). Unfortunately it looks like the thermostat housing was leaking enough to leave evidence on and around the crank position sensor. I'm guessing that the water whipping around the crank/cam position sensors at the very least wasn't helping the situation. The cold resistance of the sensor is 1.043kOhm which is squarely within the spec I have been able to find, so hopefully the sensor itself is not fouled. I will do my best to dry the belt and trigger wheel and re-fire the engine to see if the problem persists.

Yamae, is it possible to check the waveform (or at least A/C voltage) from the diagnostic connector in the engine bay or is it required to check at the ECU pins? I have been unable to find a pinout/explanation of the under-the-hood diagnostic port.

Yamae

The best way is to observe the signal at the ECU side. But it's a time consuming job since you have to remove many items. The following method can be done within the engine compartment room and is a quickest way. Insert 2 needles to the crank position sensor's wires and connect extension wires to them. The length of extension wires should be about 1 m / 3 feet or so. So that you can measure the voltage at the other end of wires. To avoid the interference of noises from ignition and injector drivers, a twisted pair wires is preferred.

fkucf10

Thanks to Yamae-san for the pointer (and highlighting of my SC400 diagram red herring) I believe I have zeroed in on the root cause(s).

As mentioned I found evidence of my thermostat housing leak around the crank sensor (in hindsight it's obvious you want to be on the heavy side of the 2-3mm FIPG bead spec in the manual), so I carefully dried and cleaned the timing belt while turning the engine over by hand. I also blew air through the bottom of the No. 1 Timing cover at the bottom of the engine which revealed some fragments of the deceased timing cover which I apparently missed in my pre-assembly cleaning (or, if I'm wrong, I've got bigger problems). I reinstalled the distributor rotors, caps, and crank sensor and fired the car (briefly, as there is no coolant in the system). Idle is back to normal, without evidence of the beat phenomenon or undue body vibration, and the tachometer registers correctly at all RPMs (that I'm willing to check without coolant/upper timing covers).

It still sounded, however, like some cylinders were not firing properly at idle, and while the bogging on return to idle after an abrupt decrease in demanded throttle was not nearly as severe as it was yesterday, the behavior was definitely still present. I pulled spark plugs 3, 5, and 7 to see if any coil/distributor issues would make themselves evident and, sure enough, found no spark on hole 7. Reinstalled all spark plugs, pulled the No. 1 coil wire off the LH distributor cap, and fired the car - same behavior as with both coils connected. Still no trouble codes, though.

So, to summarize:

• My No. 1 coil is faulty
• Something about my combination of debris and coolant in the No.1 Timing Cover made my crank sensor unhappy
• Something about the unhappy crank sensor made the ECU RPM signal unhappy above 1000rpm, and exacerbated the poor ignition conditions created by the faulty coil
• I think it is quite clear that the SC400 diagram I linked earlier is absolutely not applicable to at least UCF-10 LS400s/Celsiors
• The RPM signal at the dash in UCF-10s is derived from something other than the No. 1 coil

I think I am going to do plugs, coils, and wires. In for a penny, in for a pound. I'm sure the coils and wires are original, and the plugs are definitely over 100k (though they are the newer double-platinum Densos). I'll keep everyone updated in case further evidence of my ineptitude comes to light which could help other people avoid the same fate.

fkucf10

That is a sensible way to handle it, thank you for the advice.

fkucf10

I've now had the new coils and plugs in the car for a couple days without issue. Time will tell if I really screwed anything else up, but the symptoms which prompted this thread have been resolved.

Thanks again to Yamae for the nudge in the direction of the crank sensor, and to deanshark for chipping in so quickly.