UNDR8D Posted November 22, 2011 Share Posted November 22, 2011 Hi. I spent the entire day reading and reading before deciding to register and post. The car is a 1994 LS400 with the typical hesitation, stumbling, etc etc. I used a SnapOn Solus scanner and could not retrieve any codes. Furthermore, the CEL does not illuminate at KOEO. Also, manually jumping with a wire produces no result. THe CEL comes on while running and does occasionally flicker. Underhood diagnostics: ignition coils have full and strong spark (ouch), and there is spark to all cylinders. All plugs are dirty but otherwise ok. I am ruling out a fuel or major mechanical problem because I have been doing this for long enough to know the difference. From what I have read the rough running with the CEL failure/flickering and no codes, typically indicates a bad ECU. I removed the ECU and opened it up and it looks clean but of course that doesn't mean it has not failed. I believe it needs an ECU. Just looking for some kind of confirmation from the community before I spend $200ish on a reman. Quote Link to comment Share on other sites More sharing options...
curiousB Posted November 22, 2011 Share Posted November 22, 2011 Search the forum regarding broken wires in the trunk hatch hinges. It sounds odd but damage to a couple of those wires (copper breaks inside the plastic jacket) can lead to erratic behavior of the vehicle including the dash lights and instruments. Failing that stumbling in these cars is usually spark related. They have chronic problems with ignition coils failing so you might want to double check that. Could be an intermittent that occurs only once things warm up some. Coil problems won't trigger a CEL code. Crank position sensor is sometimes a complaint so you could search that as well. maybe misalignment or simply failing (intermittent). Rough idle is often attributed to dirty throttle body and/or dirty sticky IACV. This is only an idle malady so I wouldn't consider this if you have poor power at driving speeds. Quote Link to comment Share on other sites More sharing options...
UNDR8D Posted November 22, 2011 Author Share Posted November 22, 2011 I've read too many threads in which people spend $1000+ chasing problems around only to have it be something else. Those are all things that could be contributing to reduced driveability but the lack of a CEL at KOEO, no code retrieval with an expensive scan tool and no codes when terminal jumping leads me to believe it is primarily the ECU. Any response to those specific symptoms? Also, it is unlikely that an ECU would fail in those regards and not also effect running condition... Also, to edit my original post, it is not just running kind of poorly. This morning it all of a sudden went from running fine to extreme power loss, major hesitation, no acceleration, etc etc., however did not completely die. Also, pulling the plug wires off individual cylinders produces no effect and the spark is weak, leading me to believe it is currently in limp mode. However, it should still display the CEL at KOEO and should put out codes. Quote Link to comment Share on other sites More sharing options...
curiousB Posted November 23, 2011 Share Posted November 23, 2011 I've read too many threads in which people spend $1000+ chasing problems around only to have it be something else. Those are all things that could be contributing to reduced driveability but the lack of a CEL at KOEO, no code retrieval with an expensive scan tool and no codes when terminal jumping leads me to believe it is primarily the ECU. Any response to those specific symptoms? Also, it is unlikely that an ECU would fail in those regards and not also effect running condition... Also, to edit my original post, it is not just running kind of poorly. This morning it all of a sudden went from running fine to extreme power loss, major hesitation, no acceleration, etc etc., however did not completely die. Also, pulling the plug wires off individual cylinders produces no effect and the spark is weak, leading me to believe it is currently in limp mode. However, it should still display the CEL at KOEO and should put out codes. Sounds like you've made up your mind on the problem. Not sure why you posted here. FWIW my suggestions won't cost much if anything. Further ignition coil is not ECU detectable hence consistent with your situation of poor running and a known problem with these cars. When a coil fails the car runs on 4 cylinders. It runs but horribly, but it runs. If you leave it long enough one side exhaust will glow red hot from unburnt fuel passing to exhaust. Pulling plugs on the dead side will do nothing. Pulling plugs on the good side will result in increasing roughness and eventually stalling. If you'd rather swap a $200+ ECU on a hunch instead of a $20-30 coil go for it. Have fun. Quote Link to comment Share on other sites More sharing options...
SRK Posted November 23, 2011 Share Posted November 23, 2011 I know that the OBDII Lexus have a code for ECM failure. They will condemn themselves and report it. I don't know about the earlier systems, but more than likely they will too. Bosch systems can of the early 90's can do it. There is no "typical" stumble for these engines. However, as B points out a bad coil can produce a misfire, or fail completely. Your test proves that both coils will produce a painful spark. However, one of them may not be capable of firing at the rate required by the engine. No human can detect an intermittent spark by "feel". If you have access to the old style ignition oscilloscope you can hook it up alternately on the two coils and observe the secondary voltage trace. Each coil feeds two cylinders on each bank, and the engine will run well enough on one, or misfire on one and a "half". Limp home mode doesn't reduce coil voltage. You're right, OBDI doesn't monitor everything. But with bad spark plugs and on/off misfire, I'd be looking at the secondary ignition. Chances of an ECM failure are remote. You won't find anybody around here to support that view, no matter how much you subjectively attempt to support it. Keep diagnosing. Quote Link to comment Share on other sites More sharing options...
UNDR8D Posted November 23, 2011 Author Share Posted November 23, 2011 Sounds like you've made up your mind on the problem. Not sure why you posted here. FWIW my suggestions won't cost much if anything. Further ignition coil is not ECU detectable hence consistent with your situation of poor running and a known problem with these cars. When a coil fails the car runs on 4 cylinders. It runs but horribly, but it runs. If you leave it long enough one side exhaust will glow red hot from unburnt fuel passing to exhaust. Pulling plugs on the dead side will do nothing. Pulling plugs on the good side will result in increasing roughness and eventually stalling.If you'd rather swap a $200+ ECU on a hunch instead of a $20-30 coil go for it. Have fun. That would make sense if the coil fed one bank only but the distributors are split between both banks with a coil to each so a bad coil will effect two cylinders on each side. Not just one bank. Quote Link to comment Share on other sites More sharing options...
curiousB Posted November 23, 2011 Share Posted November 23, 2011 Yes you are correct 2 cylinder/side to each coil/distributor. Same point holds though a dead coil or igniter will take out 1/2 the cylinders and is a frequently cited problem on this BB. I don't have this problem because my '04 is coil over plug ignition. Quote Link to comment Share on other sites More sharing options...
fsuguy Posted November 23, 2011 Share Posted November 23, 2011 I agree with what curiousb points out. Also, I have experienced the exact problems you have described - twice, and both times it was the coil. Although, there is a good possibility of the ECUs going south - the capacitors used in these models appear to have limited survival based on some pictures of open ECU cases and circuit boards that I have seen. Consensus in some circles is that after about 15 to 18 years, the caps need replacing. In your case, if you have opened up the ECU case and have not noticed misshapen or leaking capacitors, then it is all the more reason to check your coils - first the driver side one, then the other. Also, the FP resistor may be on the way out. If you do check the resistor, do so when it is hot...and let us know what your final solution is!! HTH Quote Link to comment Share on other sites More sharing options...
jaswood Posted November 23, 2011 Share Posted November 23, 2011 Absent a CEL I would suspect a failure of the catalytic converter's internal honeycomb structure. Falls apart, often, and intermittently, blocking exhaust flow. Idles okay, but any power level requiring high exhaust flow.... You get an intermittent CEL since the secondary, catalyst efficiency, O2 sensor goes out of range but only with a serious level of exhaust flow blockage. Glowing exhaust pipe/converter is sometimes a clue. Quote Link to comment Share on other sites More sharing options...
UNDR8D Posted November 24, 2011 Author Share Posted November 24, 2011 Absent a CEL I would suspect a failure of the catalytic converter's internal honeycomb structure. Falls apart, often, and intermittently, blocking exhaust flow. Idles okay, but any power level requiring high exhaust flow.... You get an intermittent CEL since the secondary, catalyst efficiency, O2 sensor goes out of range but only with a serious level of exhaust flow blockage. Glowing exhaust pipe/converter is sometimes a clue. The CEL is not intermittent, rather, it "flickers" intermittently in conjunction with the misfiring. This indicates to me that there is a short of some sort that is related to the misfire and the flicker/inability to pull codes/inability to jump terminals. Quote Link to comment Share on other sites More sharing options...
UNDR8D Posted December 21, 2011 Author Share Posted December 21, 2011 For anyone experiencing these symptoms, it was indeed the ECU that had failed. I sent it out for a rebuild and the car runs great, ceteris paribus. Quote Link to comment Share on other sites More sharing options...
fsuguy Posted December 23, 2011 Share Posted December 23, 2011 UNDR8D, Glad you got it resolved and posted your findings. Can you let us know who did the rebuild/repair of the ECU for you? TIA Quote Link to comment Share on other sites More sharing options...
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