An Engine Idling Troubleshooting Question worthy of "Car Talk Puzzler"

Maintenance/Repairs
1

-1993 Toyota Camry LE

  • 4 Cylinder / Automatic w/277,500 mostly trouble-free miles

OK, I’ll give this a try. Both my independent Shop with over 35 years combined experience between the two techs
and the local Toyota Dealer can’t figure this one out.

Start the Cold Engine in cool (55F) Weather and the fast idle goes up to approx. 1100rpm while it warms up. (Normal)
After the Engine Coolant Temp reaches approx. 1/4 (about 2 minutes) the Engine Idle slows to approx. 900rpm. (Normal)

Further idling after 4 minutes; the engine idle will slow to between 450/600rpm and starts “surging” and idling very erratic. (NOT Normal)
It won’t stall, but comes very close to it and idles very rough and continuously surges at very low rpm like it’s starving for Air or Fuel.

If I drive it and get the Engine Coolant Temp. up to approx. 1/2 (Normal) it will run smoothly until I slow down to an idle again, like at a stop light.

Putting the Transmission in Neutral or Park has very little effect.
When the engaged Transmission load is taken off the Engine, it idles a little faster @ 600rpm, but still surges.

NO “Check Engine” light ever comes on and the ind. shop (or Dealer) cannot read any OBD Codes

What has been done so far:

Spark Plugs and Plug Wires have been replaced
Airbox and Filter Checked (OK)
Throttle Body/Plate and TB Interior was Cleaned
Fuel Pressure was checked (OK)
Fuel Filter was checked (OK)
Dealer tech. said ‘maybe a clogged EGR but you have to replace the IACV first to be sure’.

I followed their X-pensive suggestion and paid the $98 “Diagnosis” Bill which I now regret.

I was skeptical of this "Replace the more expensive IACV first and then try the less expensive EGR so I had the EGR CLEANED first — It was dirty/carbon residue on the internal Valve Seat so the EGR Valve was replaced to be sure.

Idle Air Control Valve (a $320 part !) was replaced

No difference even after BOTH IACV and EGR were replaced. — Disappointing…

EGR Vacuum Hoses were all Checked for leakage (OK)
EGR Metal Tube was checked for Clogging (OK)

Chevron “Techron”, Lucas Fuel Injector Cleaner, and ‘Sea-Foam’ have been tried as additives / cleaning agents
with just a slight improvement but still Idles very Rough @ approx. 500~600 rpm.

What could be causing this ?

Are there any other Sensors which could be giving false input to the ECU to cause this Idle problem ?

So far I’m out over $800 and the problem isn’t solved…

Thank you in advance for any suggestions.

david

2

The erratic idle and surging are indications of a vacuum leak. These mechanic have been throwing expensive parts at the car, and the solution may be a cheap $0.10 vacuum line or a still cheap $20 intake manifold or plenum gaskets. Have any of these been eliminated as the problem? The ODB system in this pre-1996 car wasn’t designed to detect vacuum leaks or misfires.

3

EASY test if busted knuckles is right, is to spray the area around the intake with starting fluid, if the RPMS climb high and fast you found your leak.

4

How bout the MAF? Was it cleaned or looked at? What you have listed are definitely good culprits… Hmmm not too sure off the top of my head, I’d have to think about this one… It’d help GREATLY if the shops had a snap on “scope” to diag the engine while running…it can and does reveal A LOT of useful info…

Vacuum leaks cause a HIGHER idle…if you spray a vac leak…the RPM’s will drop… but not sur what happens when you spray it with starting fluid…that’s not a great idea actually…to flammable… Most guys use water or a light spray oil to do that test.

5

The first step on an engine performance problem should be a compression test and especially so on a car with well over a quarter million miles.
A shop should make absolutely sure they’re dealing with a solid engine before even considering the replacement of one simple, inexpensive part.
It’s quite common to throw parts at a vehicle en masse and then discover, after the fact, the motor had a mechanical issue that makes all of those part replacements worthless.

If the compression is down on even one cylinder this can have a dramatic effect on engine idle, performance, and fuel mileage.

Offhand, and assuming the engine is good mechanically, an erratic idle often points to a vacuum leak or an Idle Air Control valve problem. Checking for a vacuum leak is kindergarten simple with a vacuum gauge and a compression test is not much more difficult.

6

Vacuum Leaks have been checked by lightly spaying less volatile WD-40 around Throttle Body and Intake Manifold interfaces to Cylinder Head while Idle was steady. Idle did not change

All Vacuum Hoses were carefully removed individually and pressure checked while flexing them under water.
No Leaks there either.

Sensors usually throw codes when they go bad, or even “intermittent bad” like this Idling problem.
Check Engine Light never comes on and there are no Codes picked-up by the OBD scanning device.

One Sensor I may buy and instal myself is the Engine Water Temp Sensor which sends a signal voltage level to the ECU when the Engine Coolant is Cold causing the ECU to increase the FI’s Pulse width to richen the Fuel level to keep a cold engine running until it warms up. What if that Sensor never or improperly sensed the warmer temperature and kept the A/F Ratio too rich for a fully warm engine ? Would hat cause low rpm “Surging” ? (I think) it also sends a Signal to open the Throttle Body IACValve.

Will check compression tomorrow. I have a high quality Analog Gauge and a Digital back-up Gauge.
What should the Compression be for this 4-Cylinder 2.2 litre engine ? (COLD)

7

Checking for vacuum leaks by spraying a fluid is not really an accurate means of determining if there is a leak. Vacuum lines often run off into other areas such as the dashboard, the carbon canister, etc. and which are not part of the engine asssembly.
A vacuum gauge can be connected in seconds and it will reveal instanty if there is a vacuum leak or even an engine mechanical problem due to compression.

It’s also quite possible to have a faulty component and with no code being set.

On a very good engine is great shape the compression should be in the 180-190 PSI range. Given the high miles of the engine I’d say if the motor was carrying 160 on all cylinders then it’s doing pretty well. Look for consistency in the readings because if you have 3 at 160 and 1 at 120 there will be a problem with idle.
Hope that helps.

8

I guess I have taken fairly good care of this car — Cold Compression Numbers below:

COMPRESSION:

#1 206 #2 192 #3 195 #4 208
Using a Digital Gauge (from left to right on engine)

#1 185 #2 170 #3 178 #4 185
Using an Analog Gauge

Number 2 Seems a little low but overall OK—

I also have a quality hand-pumped Pressure/Vacuum Gauge (Mity-Mite brand) but not sure which
Vacuum Line to Connect it to, or if the other end of the line being tested needs to be removed
and clamped off or if to just leave it connected to whatever its already connected to.

Lots of Vac. lines to choose from here, and as you said, some of those lines disappear into the engine compartment.

Is the Green (Middle) Sensor the EFI Water Temp. ?

9

I checked all the vacuum lines end-to-end with Vacuum and Pressure.

All OK—

10

It looks that way…for the green sensor that is…

BTW…where does your VG30DETT reside?..In one of my other favorite sports cars of all time methinks? Twin Turbo 300Z perhaps? Mmmm

Blackbird

11

The compression is amazingly good for the mileage and that should not be the cause of the erratic idle.
My point about a vacuum leak is that the problem could be due to a ruptured diaphragm in a dashboard vacuum pod or something of that nature. Some heater box doors are vaccum operated and a leak can exist anywhere inside the dash or even the carbon canister system.

The easiest and best way to know if there’s a vacuum leak is by connecting a gauge to an intake manifold vacuum nipple. If there’s a problem then it’s a matter of pinching off hoses one at a time until the gauge reading shows normal.

Vacuum readings will vary based on a number of factors but with good compression you should generally see about 17"-20" of vacuum at idle with the needle remaining rock steady. Any lower than 17 and/or a quivering needle means a problem.

For what it’s worth, there’s a multitude of things that can cause a rough idle and even a single weak valve spring can do it. (This is something that should show up on a vacuum gauge also. It’s not common but can happen and does make the point about obscure problems.)

12

VG30DETT

Yes, in my 1996 Twin Turbo 300ZX— about a 9.90/10.0 in all respects and “Assembly-Line Stock”

13

Taking a Vacuum test right off the Manifold sounds tricky since a few of the items all those lines connect to won’t hold a Vacuum (or low pressure from a hand pump) by design. All the lines themselves have been checked end-to-end. The Heater and Vent Controls in the car are mechanically actuated I think–I’ll check this. Can you be more specific of what Vacuum Line function goes through the firewall and under the dashboard ? I’ve been under this hood for a couple days and am getting real familiar with the layout and haven’t seen any Vac. Lines going in that direction.

After I check the interior of the Distributor/Coil Assy. for a burnt Condenser I think I’ll replace that EFI Water Temp. Sensor (Green one on the Left). I have been told that can send a intermittent bum signal to the ECU during warm up which can keep the Idle Air Control Valve open. The problem is, the diagnostic time it takes to set up an O-Scope to monitor it would cost more than the Sensor itself.

14

Did you get my PM? I was having trouble posting here at that time.

Your system uses either the TPS (throttle position sensor) or an IPS (idle position sensor) to tell the computer to control the idle via the IAC motor (IACV). If you have an IPS, it may be known by another name, if you don’t then the TPS serves that function.

I suspect that as things heat up under the hood, the idle position signal is getting lost. The only thing left to control your idle will be the throttle stop screw. Since your idle speed in neutral is about 600-650 rpm, I don’t think anyone has messed with it, and they should not mess with it under any circumstances.

Who ever you paid a diagnostic fee to should have checked these parts, but I’m guessing that since your car is pre OBDII, they didn’t know how to do any troubleshooting on non OBDII vehicles.