jimskelton

Hi,

I have a '91 LS400 with 80K miles on it. After driving it a while, especially on the highway, I've found it has a very specific misfiring problem. At first, I thought it was rpm related, as it seemed to show up only between 1500 and 1900 rpm with the throttle around 5-10% open. But I think the rpm is circumstantial, because the rpm is rarely above 2000 with the throttle under 10%. So now I'm beginning to think it's fuel/throttle related.

The misfiring is not serious, but can be felt slightly, and maybe occasionally with a bigger hiccup. It never occurs when the throttle is pressed down more than around 10%. Typically it will occur when travelling 50-55 mph on a flat stretch of road (with the throttle pressed around 5-10%). Once I get to 60, the throttle is pressed down more and it does not occur, unless I'm going down a slight hill and I let up the throttle to that same range (5-10%), then I can just barely feel it. There are no power issues when going up hills. Idling is very smooth, and is around 600rpm warmed up and 900 with AC on in neutral.

I just changed the plugs and air filter but otherwise, everything else is original. This problem was probably there when I bought it a month ago, but we live in a small town, so it didn't show up until we just took a trip in it. We drove it inland where it's drier and it may have been less pronounced. Back at the coast here (rain, humidity), it may be worse, but it's hard to say, which would point to spark plug wires/distributor/rotor.

I was going to change the plug wires to see whether that is the problem, but it seems strange it only happens at a very narrow range (5-10% open) in the throttle.

Also, when I rev the engine in neutral. there is a rough spot around 1700 rpm (actually, it starts getting a little rough at 1500 and goes to around 1900), but it otherwise runs very smoothly at all other rpms. No codes are in the ECU.

Any ideas besides plug wires??

Thanks for any help.

PureDrifter

iTrader: (10)

check plugs/wires, and make sure the TPS is calibrated properly (as per tutorial at www.lexls.com)

jimskelton

OK, this has been a while, because this only happens on the highway and I don't get on the highway much. I checked and double checked the TPS and it needed a bit of adjustment, but nothing much. The MAF is good, and I checked the plugs and wires to see if there was any serious misfiring. Everything looked good.

By pure coincidence, I just got a Previa which has a very similar problem, except it misfires worse on the highway. I looked at the TPS on that, and adjusted it, but to no avail. I changed the TPS with my parts car, changed the MAF, no difference. Then I checked the EGR. The vacuum tubes were routed wrongly! Vacuum was routed directly to the EGR, instead of through the modulator. I routed it correctly and now there's no problem.

So I went back to my Lexus and as a test, disconnected and plugged the vacuum to the EGR, took it out on the highway--NO MISFIRING. I cleaned and checked the modulator, made sure the bsv was working correctly, made sure everything was routed correctly, tested the EGR system as per manual, and everything checked out. It still misfires when connected. Basically, there is too much exhaust gas entering the manifold at a certain throttle position/RPM. I haven't removed and inspected the EGR yet, since it appeared the EGR was fine (the spring/valve seemed to work OK with "mouth vacuum" when sucking on the vacuum hose connected to it). Could it still be the EGR valve that is defective? Or the modulator?

I had a hard time understanding exactly how the EGR system worked. There's quite an intricate interaction between manifold vacuum/throttle value position, and exhaust gas pressure. It can see that the modulator regulates the EGR valve, with two vacuum hoses connected to different positions on the throttle body, but it also has a connection to the exhaust gas on the EGR as well...I'm thinking the modulator may be at fault, but that seemed to test out fine as well.

Does anyone have any ideas as to what the problem in the EGR system could be? Disconnecting it probably isn't an option, right? How would affect the car if it was disconnected?

ancdmd

Can you describe how the problem sounds on the highway? I have a similar situation with my '02 LS430, it has a terrible pinging sound most noticeable at higher speeds, maybe it's the same thing going on. I'm going crazy trying to figure it out...

https://www.clublexus.com/forums/mai...g-me-nuts.html

jimskelton

This is a misfiring problem characterized by the car slightly jerking/lurching as it misfires with no specific sound. It's felt more than heard. Yours seems to be almost a ringing sound.

If there's anyone with a good understanding of the EGR system, that would be great. Especially with how the electric bsv is integrated into the system. I figure the bsv releases vacuum to the EGR when it's no longer needed, which may be the problem, though it always seems to run fine at idle. Applying a vacuum to the EGR at idle will stall the car, which they say is normal.

j2b4o

there is a good link at the bottom of the thread that helped me understand and helped me test the system.

https://www.clublexus.com/forums/ls4...alve-bvsv.html

Healthserv

sounds like it is related to the lockup of the torque converter

jimskelton

Definitely not the torque converter lockup since this misfiring problem goes away when the EGR is disconnected. The article posted by j2b40 though has a link in it that I've reread a few times and it gives some very good information about the EGR system. One idea that didn't occur to me is that perhaps a resricted exhaust system may be creating more back pressure and opening the EGR through the modulator, since that also monitors exhaust pressure as well. I'm going to go through my system with that article handy and do the checks outlined in the article. Hopefully some clue will show up.

At this point, I'm treating this as an EGR valve allowing too much exhaust gas into the manifold at a very specific throttle position and RPM.

YagerD

My LS has 145k on it and has regular maintenance its whole life. I've driven it for 7 months now, a lot of city and highway often as well. I have had this hiccup ONCE and it was a fairly large one. I didn't see the exact rpm drop but it felt substantial. I was cruising on the freeway and barely stepped into it when it happened. If you figure it out I'd be curious to hear what it is.

fixmiester

Could possibly be injector related; if one or more of your injectors are partially plugged, or your fuel pressure is low, the fuel delivery at low/partial throttle could result in poor combustion and engine studdering, especially if a little exhaust gas is introduced to further slow combustion. Wouldn't hurt to try an injector cleaner before spending money on more expensive stuff, like EGR repairs.

jimskelton

OK, I will try that. I figured if it was the fuel injectors, I would see performance issues throughout the power band, and there are no misfiring or power issues anywhere but described. But I can see your point, that partially clogged injectors may not be delivering enough fuel at 5-10% throttle, but may deliver enough above that, so I'll get some high quality injector cleaner and try that.

Thanks for the idea.

cobalt91

It sounds like an egr issue to me. You said you checked the valve. How is the conditon of the vacume hoses goin to it?

mspearl95

distributor(s). if it does it within a certain speed/rpm range.

jimskelton

Thanks for the ideas. I'm not sure what would be wrong with the distributor to cause an RPM/speed issue. Would that be a crack in it?

I'm getting more convinced it's an EGR issue, though it may also be injectors. I just took a trip on the highway and plugged the P port on the EGR modulator and the vehicle ran very smoothly with no misfiring issues at all. I figured that plugging just that port would stop EGR gas from entering the intake between the 5 and 10% throttle range, since the P port is connected to the E port on the throttle body, which delivers vacuum when the throttle is open around 5%. Unfortunately, Code 71 (EGR) was thrown, which surprised me. I checked, and found that the exhaust pressure tube on the EGR modulator was disconnected (probably happened when I removed the modulator for inspection and cleaning). So that would mean the EGR wouldn't ever open.

So I connected the exhaust gas pressure port to the modulator I'm going to try again plugging the P port and see what happens when taking it on the highway. For good measure, I also put a bottle of slick 50 fuel system cleaner in to see whether cleaning the injectors will help.

I've researched the EGR system quite a bit and it is quite complex. I've checked the whole EGR system according to the manual, and each component seems to check out OK. I think my next step is to buy a vacuum gauge and do a road test. According to the manual, EGR is disabled by the VSV:

--when the engine is cold
--during deceleration (throttle closed)
--light engine load (air intake very small)
--engine speed over 4000rpm
--when the engine is racing (transmission in neutral)

The last point means the EGR system can't be tested in the shop. Grrr. In fact, the conditions for the ECU to throw a code 71 are:

--Engine warmed up
--transmission in at least 3rd gear
--speed 45 - 70mph
--EGR gas temp is under 65 degrees Celcius when driven for 90 seconds or more.

I'm now wondering about the 3rd criteria of the VSV being activated (thereby venting the EGR, turning it off): light engine load. How does the ECU detect "light engine load (amount of intake are very small)"? Would this be through the throttle valve position sensor? If so, I think I'll double check that and make sure it's set for factory specs. It says .040 is necessary and I may have reduced that...

Anyway, I feel like I'm getting closer, but the system seems to be operating as specified in the manual. I'll keep you posted.

kbconv

While my symptoms weren't exactly the same as yours (mine wouldn't go past 1800-2000 without stumbling when I tried to accelerate), I wouldn't trust the stock ECU if you're having odd fuel/ignition type problems and haven't had the capacitors checked in it. All 8 of the Nichicon brand, PF series electrolytic caps in my car's ECU were leaking to one degree or another.

This is what convinced me to open mine up. - http://forums.club4ag.com/zerothread?id=87031 Also take a look at www.badcaps.com

After replacing my fuel filter and still scratching my head, I came across some posts about ECU problems and decided to open mine up, sure enough about 3 of the 8 caps were leaking quite visibly onto the circuit board. I got a replacement cap at Radio Shack for the worst one (a 10uF I think), soldered it in and then tested it and it would rev right up to 5K with no problem. (cost - $1.39 plus supplies).

They didn't have a replacement 15uF cap so I decided to order 8 - 105degC caps (from a major online electronic parts supplier) of the series that is the replacement for the obsolete series of caps found in my (our?) ECU computers. I replaced them all last week but don't think I noticed any difference over just replacing the one nasty one I discovered, but the others would have failed in the future I'm sure. FWIW, the PJ series replaces the old PF series from Nichicon, I just had to order a little higher voltage in maybe 3 of the caps to stay in the same series with all of them, but the tiny extra bit of lead spacing on them caused zero problems when I replaced them.

So, it may or may not be your issue and I'm not saying replace to run out and replace your ECU, just wanted to mention it and it just takes an hour or so to remove it and open the cover to look on the circuit board for signs of leaking caps. Also, it doesn't appear my failure was an isolated incident, there appears to be a trend of cap failures going on in some electronic parts. I have 177,000 on my car but the cap failures are probably more a function of age and heat rather than mileage.