davemack

I've been chasing an irregular idle problem all summer. I've replaced the O2, IAC and TPS sensors, checked all vacuum lines and gaskets and still get an intermittent idle surge. At normal op temp, it'll idle between 600-700 rpm, then rev up to 900 rpm for about 5 seconds, then drop back down to 600-700 rpm. It rarely stays at any specific idle speed (at least on the numeric digital readout); it usually wanders between 600 and 700 rpm, but sometimes higher or lower.

Last, it often seems to "lope" at the lower idle, and usually smells rich (I have the Bosch D9B injectors - 22 lb Ford's, which might mean I need to lower my FP). Ideas? I still lean toward a vacuum leak, but I'll be darned if I can find it. I don't have a FP guage, so haven't checked that. Been thru several diagnostics, the last said he thinks I may be getting too much fuel, but otherwise everything is spot-on.

FrankN

Mine did the same. Had to replace one of the two idle control motors.

davemack

By idle control motor, do you mean idle air controller (IAC)? Are there two?

kopbet89c4

Your EVAP system could be causing it. I eliminated mine and there was no more idle surge after all. My car would start up and idle at 1400 rpms right away, then down to like 500 and up to 1200 and down. Did it until car was fully warmed up. Then idle would normalize at 700.

If you have inspection/emissions testing in your area, I'd fix it instead.

davemack

MN tried emissions testing, but abandoned it. So, what is an EVAP system and how would one go about eliminating it? I wonder if the 85 has the same as the 89? I'll need to look in my FSM.

mikey whipreck

Hook up a vacuum gauge and see what your car is pulling.

kopbet89c4

To disable the EVAP system, locate the 2 ports on the throttle body.

There is one tiny small port underneath the TB and one under the PCV hose. Put some rubber cap plugs on them.

That is all.

If you still want a functional EGR system, you can just use the port under the TB as the EGR vacuum access port so you don't possibly get a SES code 32 for the EGR system.

rick lambert

84s have 2 IACs....all TPIs only have 1.

davemack

That's what I thought, but wasn't positive.

Some additional facts; the car will not only surge, but will also drop into a low idle - around 500-570 as well. This seems to happen almost anytime I put the car in reverse when at op temp. It seems to shake quite a bit at idle as well. I tried disconnecting the MAF as per the FSM, but it idled much worse and responded slowly when revved. SES light also came on and stayed on.

I do need to check FP, no doubt. I also know I have an intake gasket oil leak in the rear that could be the source of a vacuum leak. But, considering it's random, sometimes surges, other times idles slow, sometimes is smooth as butter at idle and other times shakes quite a bit, sometimes is slow to return to normal idle as I slow down on the road and other times seems to return to idle easily, I wonder if it's the ECM? How about the alternator? As mentioned, new TPS, new IAC, TB was rebuilt last summer (mechanic sent it out for this), new air filter, new Bosch D9B injectors (I had a leaker), no other mods or fixes of late.

Help!

rick lambert

I would definetly start checking for vac. leaks. It's a slow tedious job, don't only check the connections...but check the entire length of the vac lines. You can use alot of different products for this...an UNLIT propane torch, starting fluid, or carb cleaner, even WD40, but it's messy, if you hit a vac leak the RPMs will change!!!

davemack

I have sprayed all vacuum lines, gaskets, etc. with intake cleaner, no leaks detected. Very good advice, though, and still maybe the culprit.

I forgot to mention, I have also changed fuel filter, spark plugs/wires and O2 sensor. The plugs/wires were old and the O2 sensor cost all of $25, so I figured it was worth a try.

I've also had the car diagnosed by a mechanic both at idle in his garage and while driving around. He tells me it's fine. No way, it's not fine. Starts at around 1200 rpm cold, drops to about 900-1000 for about 2 minutes, drops to around 800 but only after revving a bit, then sits at about 680-750 rpm. If I allow it to idle, it will occassionally rev up to 1000-1400 rpm for about 3 seconds, then drop back to 700 rpm range. If I drive it, it will idle as described before - pretty much at random with a mind of it's own.

The D9b's are Ford injectors and were specced at 22 lbs at 30 psig (I believe Ford runs at or near 30 psig FP). The mechanic that did the doagnostic work (one of Corvette Fevers 100 best) said he believes it is "getting too much fuel", but didn't bother to check FP (just used a laptop sized computer diagnostic device).

Will try to measure vacuum. I might have to bring it in for that, though. I don't own either FP guage or vacuum device. I will run to the auto store tomorrow and try by-passing the EVAP as suggested by KopBet89c4. Is there just one size vacuum line? I'm thinking about just replacing all with rubber (I've read others have done that). Anyone know off-hand what size line I need, and about how much for an 85?

Thanks all for the great advice - I really appreciate it!

mikey whipreck

Vacuum Gauges are cheap, and could be worth it. You could pick up a fuel pressure gauge there too. It's hard to figure out idle problems without any numbers. I tried that.

I don't have my FSM off hand, but the 85's are unique in fuel delivery for L98's. They run from 34-39 psi with the pump energized and a drop of 3 - 9 psi at idle. I seem to remember the injectors being 24 lbs, as opposed to later years that are 22lbs...

Check the cold start injector as well...

kopbet89c4

Try capping off the 2 ports that go into the EVAP system first and get back to us.

Then try resetting the base idle and TPS voltage. Mine is set at .6 volts with a minimum idle speed of 600 instead of 400.

jmrl98

After all of the above diagnostics, your alternator might be another possibility.

davemack

Great advice guys. I will look into a FP gauge and vacuum tool tonight. I will also try by-passing the EVAP. I bought some plugs at Checker over lunch today. I removed and plugged the T line under the TB, but couldn't find the other line near the PCV valve. Interesting, but plugging the T line under the TB made no difference. Is that a useful clue?

I'll also re-set idle speed to a higher rpm. One thing I didn't do when I replaced the IAC and TPS is disconnect the EST. Others have said that is a must, or the ECM just adjusts timing to maintain the memorized idle. But, after replacing the TPS and re-setting the old IAC, timing was spot-on when the mechanic checked it out.

As for the cold start valve, would that affect idle, or just start up? It starts immediately and runs at about 1100 rpm for a couple of minutes, then starts to back down. Strange, though, it runs at a low idle at least as often as it runs at high idle. High idle is usually a weird spike that lasts a few seconds then drops, but sometimes it does stick for a longer period, then backs down.

I also wonder about alternator. The FSM suggests checking that for rough or irregular idle... FSM also suggests testing the ECM, but shouldn't that have shown it's ugly side when computer diagnosed?

kopbet89c4

Facing to the front of the car, the right side of the TB where the TPS and IAC are, right below that bigger hose of the PCV syster is the one vacuum port you need to plug. Its a little smaller in diameter. That port also leads to the EVAP system/canister. Plug it and see if it solves the problem.

davemack

Sorry guys - had to mow the lawn before I could play with the car..

Okay; on my 85, there is a T fitting under the TB. It has the vacuum line that goes to the evap canister and the vacuum line for the EGR solinoid coming into the horizontal tube of the T, and the vertical tube of the T goes into the TB. kopbet89c4 mentions two separate fittings, not one fitting with two lines coming into a T. If there should be two fittings, we may be onto something, since both vacuum lines enter the same fitting on the bottom of the TB. Only drawback remains that I cannot find the second fitting on the TB, if a second fitting exists on an 85.

By capping both vacuum lines as well as the fitting on the tb, no real change in idle. No check engine light, either. Really, no change of any significance, which causes me some concern. I may try driving it with the vac line unplugged and see if that matters.

It does seem to run between 680-720 rpm a bit more often than before (no data to support this), but still idles at 550-650 plenty of the time with the occassional spike to 800-900 rpm. It also drops to around 590-620 rpm just about every time I put it into reverse (4+3 manual). And, it shakes pretty good at idle. Runs great on the open road, though.

The Hanes manual shows a vacuum damper valve where the two vacuum lines come together and enter the TB. Mine just has a simple T fitting - no valve of any sort. Is that a problem? Does an 85 even have a vacuum damper valve, or is that for later TPI set ups?

Thoughts??

kopbet89c4

So you found the bottom one. Good! Now do this one.

kopbet89c4

More help...

davemack

I have looked again, and can say for sure that my 85 does not have that second fitting on the TB. It does have what looks like the spot where that fitting could be installed into the cast TB, but it's still plugged from the factory.

Did your car have a single vac line going into the bottom of the TB? Mine has two lines going into a single fitting. It looks like the EGR solinoid line and the EVAP canister line feed the T, which then connects into the bottom of the TB.

I think by just plugging the vertical part of the T fitting, I've effectively plugged those two lines. And, by also capping the TB fitting, it is sealed from vacuum leakage. Is that the same intended result? Or should I reconnect EGR solinoid and put some other vac line into the T fitting along with the EVAP canister vac line?

Thanks a ton for sticking with me on this. I know I must be driving you crazy - I'm a novice with a wrench and lack even the most basic diagnostic tools. But, we're making progress here. Maybe the fix is just around the corner..