Idle Speed Control Valve Episode - 1990 LS400
01-20-11, 08:37 AM
#1
Rookie
Thread Starter
Idle Speed Control Valve Episode - 1990 LS400
Hello CL forum members. This is my first post. I’ve been lurking here for a while. I wanted to contribute something back in return for the help I’ve gotten by researching all the great info here. By using the search feature, I was able to read many threads with contributions from helpful forum members, diagnose the problem, and feel confident enough to do my own wrenching to get my old girl back on the road.
I have a 1990 LS 400 with 225,000 miles. It was operating normally when I stopped to get takeout dinner. When I came back to the car after being in the restaurant for 5 minutes, she would start, but not run. It wouldn’t idle at all. If I held the accelerator down, the car would run smoothly, but when I took my foot off the gas, the car would immediately die.
I had my son come pick me up and then I came back later that night when traffic had thinned out. I was hoping that after the car had cooled down, she would start and run normally, but that was not the case. I was able to get it home by keeping the RPM’s up and shifting into neutral to stop at traffic lights.
After researching the CL forums, I thought the problem might be either the Idle Speed Control Valve (aka Idle Air Control Valve - IACV) or a dirty throttle body. A new OEM ISCV at Sewell is $575 but I was able to find one on eBay for $75. When I got the ISCV off the car, I tested it as per the repair manual. The resistance test came up okay. But the actuation test showed that the valve was inop. I could feel a very slight reaction as I applied voltage sequentially across the leads but it was very sluggish as compared to the reaction I got when I tested the part I got from eBay.
While I was at it, I also took off the throttle body. There was a substantial amount of carbon and tar build-up on the throttle plate, the throttle body and the intake inlet. Almost all of the build-up was downstream of the throttle plate. There are three small vacuum ports on the top of the throttle body that were fairly clogged. So I cleaned all of that.
One of the biggest problems I had in doing the work was old, brittle, hard hoses. They were a bear to get off. I had a tough time getting the throttle body off due to a fairly large ventilation hose under the TB that connects the TB to the RH valve cover. Also, getting the ISCV off was pretty difficult because it has 4 hoses connected to it. Next time, I’ll be smart enough to just cut the hoses off to make it easier to remove parts.
I ended up replacing all the hoses with OEM hoses I got from Sewell. At first, I tried to source hoses from the auto parts store. But I found that standard sizes weren’t always a good fit. I ended up just purchasing new OEM hoses from Sewell. They weren’t really much different cost wise and they fit perfectly.
While I was at it, I also took the opportunity to replace my plugs and plug wires. The plugs looked pretty good but I went ahead and replaced them with NGK BKR6EP-11 double platinums. After taking the old wires off, they all tested okay but the covers were broken. I replaced the wires with an OEM set I got from Sewell.
So after all of this, I finally got her put back together last week and back on the road. After about 75 miles or so, she seems to be running really well.
All in, I spent about $450 in parts. I’ll bet the damage would have been something like $1,500 if I’d have had her repaired at a shop - much more if taken to a dealer. I hope my experience doing these repairs and this post helps someone else.
I have a 1990 LS 400 with 225,000 miles. It was operating normally when I stopped to get takeout dinner. When I came back to the car after being in the restaurant for 5 minutes, she would start, but not run. It wouldn’t idle at all. If I held the accelerator down, the car would run smoothly, but when I took my foot off the gas, the car would immediately die.
I had my son come pick me up and then I came back later that night when traffic had thinned out. I was hoping that after the car had cooled down, she would start and run normally, but that was not the case. I was able to get it home by keeping the RPM’s up and shifting into neutral to stop at traffic lights.
After researching the CL forums, I thought the problem might be either the Idle Speed Control Valve (aka Idle Air Control Valve - IACV) or a dirty throttle body. A new OEM ISCV at Sewell is $575 but I was able to find one on eBay for $75. When I got the ISCV off the car, I tested it as per the repair manual. The resistance test came up okay. But the actuation test showed that the valve was inop. I could feel a very slight reaction as I applied voltage sequentially across the leads but it was very sluggish as compared to the reaction I got when I tested the part I got from eBay.
While I was at it, I also took off the throttle body. There was a substantial amount of carbon and tar build-up on the throttle plate, the throttle body and the intake inlet. Almost all of the build-up was downstream of the throttle plate. There are three small vacuum ports on the top of the throttle body that were fairly clogged. So I cleaned all of that.
One of the biggest problems I had in doing the work was old, brittle, hard hoses. They were a bear to get off. I had a tough time getting the throttle body off due to a fairly large ventilation hose under the TB that connects the TB to the RH valve cover. Also, getting the ISCV off was pretty difficult because it has 4 hoses connected to it. Next time, I’ll be smart enough to just cut the hoses off to make it easier to remove parts.
I ended up replacing all the hoses with OEM hoses I got from Sewell. At first, I tried to source hoses from the auto parts store. But I found that standard sizes weren’t always a good fit. I ended up just purchasing new OEM hoses from Sewell. They weren’t really much different cost wise and they fit perfectly.
While I was at it, I also took the opportunity to replace my plugs and plug wires. The plugs looked pretty good but I went ahead and replaced them with NGK BKR6EP-11 double platinums. After taking the old wires off, they all tested okay but the covers were broken. I replaced the wires with an OEM set I got from Sewell.
So after all of this, I finally got her put back together last week and back on the road. After about 75 miles or so, she seems to be running really well.
All in, I spent about $450 in parts. I’ll bet the damage would have been something like $1,500 if I’d have had her repaired at a shop - much more if taken to a dealer. I hope my experience doing these repairs and this post helps someone else.
02-09-11, 09:19 PM
#5
Rookie
Thread Starter
The ISCV is located on the forward end of the air intake chamber. I don't have a repair manual for a 1997 but here are the pages for a 1990 to get you oriented. HTH
Trending Topics
02-14-11, 10:43 AM
#12
Hey twoofanman, yea I live in the springs. I've kinda been having that issue for awhile though It only seems to shudder and die when it's alot hotter than it has been outside.
Would be great to meet a fellow LS owner here in CO for once.
Would be great to meet a fellow LS owner here in CO for once.
08-15-12, 11:46 AM
#13
Driver School Candidate
Join Date: Aug 2012
Location: england
Posts: 1
Likes: 0
Received 0 Likes
on
0 Posts
hi can anyone help me i have a ls 400 91 the idle starts in morn aroun 850 then as gets warm revs over a 1000 by its self was told could be the i acv sticking but have no photos where on car is can anyone help cheers
08-15-12, 12:54 PM
#14
Welcome to Club Lexus. 
An additional thread on the IACV:
https://www.clublexus.com/forums/ls4...rol-valve.html
An additional thread on the IACV:
https://www.clublexus.com/forums/ls4...rol-valve.html
Thread
Thread Starter
Forum
Replies
Last Post
lexus94ls
LS - 1st and 2nd Gen (1990-2000)
16
08-29-18 04:58 PM
