MacGyver!

Hey guys, I'm new to the forum (and Toyotas) and have an idle question:

When I push in the clutch while coming to a stop (or just coasting) the rpms drop down below the normal idle speed for about 1 to 3 seconds, then it returns to normal. To be clear, the idle will momentarily drop to 200 rpm or so for a second or two, before returning to 800; this normally occurs during braking when I depress the clutch while coming to a stop. The idle returns to 800 rpms without me having to give it gas. Once it gets itself back to 800 rpms, it idles fine. Other than this issue, the truck idles fine and has a very steady idle at all other times.

Any body have any thoughts on the momentary low idle?

The truck is a 1992 pickup 3vze with about 166k on it.

Thanks in advanced.

abecedarian

The ECU adjusts engine timing to control the idle speed under certain circumstances, but that requires pretty specific things- TPS signals idle to the ECU, clutch switch signal not relevant (shouldn't matter if the clutch is pressed when the throttle is closed), etc. I suspect things aren't quite right.

I suggest going through and setting the TPS, timing and idle. Jump the terminals in the check connector and set the timing to spec, then adjust the idle to spec, and repeat until both the timing and idle are to spec.

MacGyver!

So, I believe I found the underlying issue: I eventually traced the problem to the EGR valve. To test, I disconnected the vacuum line and the momentary low idle when coming to a stop vanished, when I re-attached the vacuum line it came back.

I tested the EGR valve with a vacuum pump and it appears to be operating properly. I am going to give it a good cleaning and see if that works. It seems like the EGR valve is sticking open for a moment when I let off the gas when coming to a stop.

Or maybe the vacuum to the EGR valve is too high and causing the rough idle?

masterbasch

I have the exact same issue. Let me know what it turns out to be. Thanks.

MacGyver!

So, I ran a test this weekend: I placed a t-fitting on the vacuum line going into the egr valve. I then drove the truck under conditions that I noticed the low idle and had a partner read off the vacuum when the rpms dropped (to determine if there was vacuum keeping the egr open resulting in low/rough idle). The vacuum reading should be zero when coming to a stop in order to have proper idle. However, the reading was 2 in Hg and slowly dropped to 0 in Hg (idles fine at 0 in Hg). Now, I don't know if this is enough vacuum to keep the valve open or not.

I plan on plugging the vacuum line that feeds into the egr and attaching my vacuum pump/gauge to the egr valve and pull vacuum to 2 in Hg while driving to determine if this results in the low/rough idle. If not, then I think I can assume the egr is sticking open. If so, then it seems like it is getting a bit of vacuum when its not supposed to.

If it is getting vacuum when it shouldn't be, any thoughts on what would cause this? Some sort of regulator or relief valve not functioning properly?

Thoughts?

MacGyver!

I pulled a slight vacuum on the EGR (1-2 in Hg) and it was enough to open the valve and cause poor idle. I'm going to check the VSV valve per fsm and inspect the throttle body. If this stuff checks out, I'll probably pony up for the EGR.

masterbasch

Thanks for the update, im very excited about what the solution is. i did check my EGR using the "sucking" technique and it seems to have checked out. Ive owned my truck for about 2 months now, so i am really still trying to learn the basics about it. My to-do list is already massive so this issue hasnt recieved my full attension. but im starting to get sick of tapping the gas peddle at stops to shoot the rpms back to 800, as well as my gas milage. one thing i will add is that i can hear a slight mis-fire (sometimes) while decelerating. new plugs and a fuel filter is really all i have done as far as trying to fix the problem. i was thinking of checking the timing marks on my cams to see if one jumped a tooth. Is that a possibility?

4Crawler

iTrader: (3)

Might check the dashpot:
- http://www.4crawler.com/4x4/CheapTri....shtml#Dashpot

It is there to slow the closing of the throttle when you back off the gas.

MacGyver!

4Crawler, thanks for the input. I'll check that next.

masterbasch, checking the timing certainly couldn't hurt. I checked the ignition timing before moving onto the EGR (didn't check the cam timing though). You might want to disconnect the vacuum line going to the top of the EGR valve (and plug it). Drive around: If your symptoms go away, I'd assume that the problem lies somewhere in the EGR system (the CEL will probably come on after a while with the EGR disconnected).

I checked the VSV as described in the fsm and it passed. I'll check the dash pot next (maybe the throttle is closing a bit too slowly).

masterbasch

I unplugged the EGR vacuum line and the idle was rock solid at around 800. Could be me but it also seemed to have improved throdle response.

Thanks 4crawler, I went to check my dashpot and it was gone. No wonder first gear is soo freaking jerky!!! In that case, my throdle is not closing too slowly but too fast. Does that rule the dashpot out? Im thinking it might be the EGR as far as the low idle issue. And the dashpot will solve my fast closing throdle issue. It might be starting making to make sense now.

MacGyver, im curious to know how your dashpot checks out. Thanks for your help.

masterbasch

Throdle lol. I thought it didn't look right.

MacGyver!

I checked the dash pot and it was a little bit out of spec (while plugged, upon throttle release it would hold at about 1400 rpm instead of 2000 rpm); I adjusted it to spec...unfortunately, the low idle while stopping was still present. At this point, I'm thinking I need a new EGR valve, but I want to make sure before I drop $175 on a new one.

Thoughts?

masterbasch

I just ordered one. Ill let you know if its a fix. It should come in next week sometime.

MacGyver!

Awesome, I hope to hear good news after you install. Keep me updated.

masterbasch

New EGR installed and I still have the low idle. Looks like I blew $170.

Now what?

4Crawler

iTrader: (3)

EGR should not be operating at or near idle as long as it is not stuck open. You might simply have the idle speed a little too high and the ECU is hitting the fuel cut point when you hit the brakes:
- http://www.4crawler.com/4x4/TLCA_Tru...ech.04.09.html

Or the TPS IDLe adjustment might be a bit off and sending a "throttle closed" signal to the ECU too soon and thus triggering the fuel cut.

MacGyver!

Sorry to hear the new EGR didn't fix the problem. You may want to look into what 4crawler said, but I am pretty sure my problem (which is similar to yours) has to do with the EGR system, primarily the EGR receiving vacuum when it shouldn't...so I think this is either the VSV or the vacuum modulator.

My idle and timing are correct; also, since the problem goes away when the EGR valve is disconnected (vacuum line pulled off), I think the problem lies somewhere in the EGR system.

masterbasch

Yeah I think you are right about it being part of the EGR system. Let me know if you make any progress. Thanks for the advice I think ill test my TPS system like 4crawler said just for the heck of it.

MacGyver!

Quick Update:

I finally decided to take the truck to a mechanic. He found a vacuum leak and, upon inspection, found that several of the vacuum lines had been incorrectly connected (lines going to the wrong port). It appears as though someone had removed the intake manifold and didn't properly reattach the vacuum lines (previous owner perhaps). This ultimately lead to the EGR system not functioning properly.

So, the learning experience here is to always make sure that the vacuum lines are attached and operating as intended (even though it can be a pain).

I just wanted to post the solution in the event that someone else finds it useful.

-Brian