Mustang_Eater

Hello All,

I have a 1989 Corvette. I have been working with people on here with a idleing problem when I realized that I may have been looking at this the wrong way.

I had 2 problems originally. 1) My RPMs were not steady, dropping down to 500 and the car even dieing 2) What I described as a "shake/sound" in the car like that of a car about to die

I was still trying to fix the idle issue number 2 when I realized my RPM's are stable, I have not gone under 600RPM and the car has not died again since I replaced the IAC and clean out my throttle body. However, the shake is still there.... I know believe that this is actually a firing problem at idle. Like a miss fire... the reason I say this is because I adjusted the RPM's to 800 at idle and I still have the "shake/sound".

Here is my questions,

Can a car run excellent at normal speeds without miss firing and then miss fire at idle?

Can a miss fire be inconsistent? For instance I do not get the miss fire "shake/sound" feeling every revolution. It's pretty random. Somtimes its every 8 seconds some times it's random.

What could I replace to fix a miss fire at idle?

Thanks so much!!!

BADDUCK

What have you done besides replace the IAC? Does it shake before it warms up and goes into closed loop, or only when warm?

hooked073

you can have a misfire at idle and not one at higher RPS, you can have one underload and none underload. I am lost about what you mean shake sound. I am not convinced you have a misfire because of a shaking sound noise. A low RPM Idel miss is usaly real clear to here from the exhaust. Usaly if you have a miss at idle go to the back of the car a listen for a light popping coming from the exhaust not a rattle a popping noise. These things had shakes and rattles from day one and as things wear it usaly gets worse. Now if it is real bad keep looking. But be sure you are truling missing.

Mustang_Eater

Thats whats hard to describe it. Its a "rough idle" randomly, even at 800RPM in idle. (sudden shake in the car, with the engine sounding like it just went down 200 RPM and is about to die and then going back to normal smooth idle) Happens atleast 7 times at a stop light. Maybe miss fire is to major a term, as you do not hear a pop from the exhaust.

Repalced the IAC, TPS (which I may not have adjusted correctly but It was happening before I repalced it) New spark plugs. Clean out Throttle body with carb cleaner.

Hope that is some more info to help this problem.

WW7

Has anyone suggested that you ohm your injectors yet? This sounds like it could be an injector problem..Are your injectors the stock Multecs? Let use know.When my injectors were starting to go out I had a rough idle and a hiccup every 6 to 10 seconds..WW

Mustang_Eater

Yes, they are the stock injectors. And I have used injector cleaner that I hear could break them but I had this problem since before I used cleaner in it.

How do you ohm a injector again? I will search the forumn

hooked073

Does it do it when you turn A/C off?

WW7

They could of been bad before you used the cleaner..Heres how to check the injectors::Set the meter to the correct scale, pull the cap off the injector by squeezing the metel retainer wire, put a probe on each of the prongs of the injector,it doesn't matter which way (pos or neg), if the reading is very far off of (16 ohms) you have a problem with the injector....WW

GREGGPENN

A faulty or incorrect set IAC can cause an irratic idle -- one where the idle speed goes up/down. The IAC's job is to adjust idle based on load conditions (like A/C on or off). If the idle gets too low, the motor will act like it's going to die and could shutter/shake (as you describe.) The TPS tells your computer what position the throttle linkage is in.

You have to check/set the IAC before installation and run a learning sequence IIRC. An issue with your injectors could compound the problem.


No one has attempted to answer the OP question....

Here's a basic description: Idle is controlled by the ECM (computer). Though the distributor is sending electricity to fire the plugs (just as it does on older, carbed cars), it's the amount of fuel/air that controls how fast/rich it runs at idle. The ECM decides how fast the motor should be idling by varying injector pulse width and IAC positioning. By independantly being able to vary fuel and air, speed and AFR can be controlled at idle.

GREGGPENN

If this has not been done on your car before, a pressed-in plug will cover the adjusting screw. It’s located on the driver’s side of the throttle body. Remove this plug by using a small punch. Impact the plug (lightly) on the edge of its face and this will allow its removal.

1. Start the car and let the engine warm up to its operating temperature. (It is suggested that you make sure the timing of the engine is set to specification prior to doing this procedure.) During this step, the engine should be in closed loop operation and all electrical accessories INCLUDING COOLING FANS should be off. Shut off the engine after it is warmed up.

2. Turn ignition switch to ON, but do not start engine. Adjust the Throttle Position Sensor to .54 +/- .08 volts using a digital meter. There are 3 wires stacked vertically on the TPS. You will need to be able to measure the voltage between the two top wires. You can either buy a special harness connector that breaks these wires out (from Mid America costing $12), or gently pierce the insulation of the wires with the pointy prongs on your voltmeter. You can also stick a paper clip into each of the two top locations of the connector and clamp onto the paper clips to measure the voltage. The adjustment is accomplished using a Torx driver and loosening the two screws that hold the TPS to the throttle body. Then rotate the sensor either clockwise or counter clockwise (with the ignition on and engine not running) to obtain the .54 volt specification. Retighten the two TPS Torx screws to “lock in” the adjustment.

3. With the IAC connected and the ignition “OFF,” insert an Auto Zone tool (free upon request from Auto Zone) into the ALDL diagnostic connector terminals “A” to “B.” (These are the upper right two holes in the ALDL connector.) This grounds the ECM and places it into the diagnostic mode.

4. Turn the ignition to the “ON” position without starting the engine. Wait 30 seconds before doing step 5. Doing this forces the IAC pintle to the closed position (closes air passage) within the throttle body.

5. With the ignition still in the “ON” position, disconnect the IAC connector at the IAC valve. This connector has four wires and is located on the passenger side of the throttle body. This action prevents the ECM from controlling the idle speed using the IAC valve.

6. Turn ignition switch off!

7. Remove the Auto Zone tool from the diagnostic connector.

8. Disconnect the EST timing connector located just behind the driver’s side valve cover. This is necessary to prevent the ECM from attempting to adjust the timing as you adjust the idle speed. This will be a brown wire with a white stripe and easily separates using a small screwdriver to raise the locking tab. Failure to do this step will allow the ECM to control the idle and you will not be able to properly adjust the base idle.

9. Start the engine. The idle RPM will be very low, and you may have to coax the engine using the gas pedal or your hand and the throttle cable to keep it running. I find it useful to insert a small screwdriver between the end of the idle adjusting screw and the throttle body to keep the car running at this low idle point. HOWEVER, REMOVE THE SMALL SCREWDRIVER BEFORE MAKING THE FINAL IDLE ADJUSTMENT!!!

10. If your car is an automatic, set the parking brake and put the transmission in park. If your car is a manual, leave it in neutral.

11. Adjust the idle speed screw to obtain 425 +/-25 engine rpm. This will require a Torx tool to adjust the idle screw in the throttle body. As you adjust the idle screw to obtain the desire idle speed, you are changing the TPS voltage setting. The TPS will require re-adjustment in step 12. Make sure the cooling fans are not running while you make this adjustment.

12. Shut off the engine, re-connect the IAC and the EST timing connectors.

13.Turn on the ignition switch and do not start the engine. Reset the TPS to .54 +/-.08 volts as stated in step 2. Turn off engine.

14.Reset the IAC pintle position by:
a.Depress accelerator pedal slightly.
b.Start and run engine for about 5 seconds.
c.Turn off engine for 10 seconds.
d.Start engine and check for proper idle operation.

15. The idle speed of the engine should be in the 600 – 700 range per the ECM programming with all connections re-established and engine at operating temp.

You are now in adjustment on base idle speed and TPS output. Start the engine. It
may take a few minutes for the car to “catch on” or learn its new settings.

Revised by Bob Lamb 10/03 from a tech article written by Lars Grimsrud 10/01. These changes were required to align the Grimsrud article to the Helms GM manual for an 89 Corvette.

GREGGPENN

You could also have a vacuum leak (that started before you replaced any of the parts).

Take a look at this current thread for a bit more info....

http://forums.corvetteforum.com/c4-t...-any-tips.html

Mustang_Eater

Ok, I'm going to try the reset. I just understand how it could be a idle adjustment problem since it happens at 600rpm and at higher 800rpm idles...

One interesting thing I have noticed, when I get the Idle stumble problem the whole dash flickers a little right when it happens.... not sure if that helps....

GREGGPENN

Check the alternator and connections.

TheFinn

Guys,

would the idle setting method described above in Gregg's Post #10 suit for an -86 as well?

dunno513

Just finished this on our 89 where we had thoroughly cleaned the TB and IAC and intake runners and replaced injectors. We first noticed a high rpm (2200+) on start up and the fact that the TPS was maxed out in its adustment range.

We ultimately had to back down the idle about 200 rpm. What we used as a trick for testing the voltage was drilling two small holes in the connector of the TPS for posts A & B. The holes were just large enough that our tester points fit snug and made good contact. No juggling while adjusting. Afterwards, the TPS was in the middle of it's range, and the startup surge is now only going to 1300'ish and backs down to 800 and settles at 700.

We plan on doing this again after we get it timed and while using a more accurate rpm sensor. 89 tach is not exactly an accurate instrument. We backed down the idle until we saw it hit 4 figuring it was probably 450 in reality.

Process a little intimidating to read, but once you finish and understand why you do certain steps, it's actually really easy.

Oh, and the brown wire with white tracer is most likely a brown wire with a black tracer on those less than pristine cars.

JAKE

My take on this:

As my Standard Rule, I address the most probable cause first before moving on to anything more advance, difficult or costly to do. Basically, what I do is take the logical, sequential approach moving from probable (cause) to possible. Many times it turns out to be something relatively simple to fix.

In your case I'd classify it as a random misfire.

I'd first check the firing of the plugs. This is a simple thing to do. I use my timing light and check check each plug wire for even steady flashes on the light as the engine. If you don't have one, auto parts stores sell an inexpensive substitute. In intermittent NO FLASH tells me no voltage is, for that instant, travelling down the wire. Finding out why becomes the next task.

I also do the Total Darkness check to see if any spark from the wire(s) or plugs is jumping to ground. My son just found such a condition on his 96 LT1. Re-positioning the spark plug wire boots solved that. He changed plugs and re-installed one of the plug wire boots too close to the exhaust manifold. Even though the wires are high-end and only 1-year old, spark still found it's way to ground in an intermittent fashion. Spark plug boot clip not affixed to plug end. Remember "path of least resistance".

Hope this helps.

Jake

AGENT 86

You cannot adjust the idle, that is controlled by the ECM via the IAC. If you try and crank up the idle with the minimum idle screw, you are also changing the TPS idle voltage. If that is way out, you could have a wonky idle.
Follow the write up that Gregg posted and get the min idle and TPS idle voltage set up correctly.

As pointed out, OHM test those injectors.
Jon has a video here >> http://forums.corvetteforum.com/c4-t...questions.html

lopezisgod

not stealing thread.. just putting in my two sense.. with my situation..

did all what was said here..

have an 86 tpi maf system..

replaced:

o2, tps, iac, ohmed hot and cold on injectors all 16.4-16.7

cleaned throttle body extreamly well..

it has a custom chip and egr, smog, cold start injector removed..

is an 86 comp with 87 motor( roller),

i have an coolant temp sensor for the computer to put in as i was told this may be it as well..

i have an old snap on "brick" scanner and can watch the reading when it runs

my integrator is 128
block learn 150
blm count 0
iac 0 (concern?)
maf 7g/sec (read that 6-9 is norm at idle )

this happens weather its in closed or open loop..

i also notice a strange issue.. on WOT... when it hits liek 3800 rpms.. the tac starts to go back wards back down... but still is climing in rpms..

also i notice.. if i take a hard left turn and get on it full throttle.. the motor just dies... i let off..and bam motor sounds like its running again..
litterally like i just turned the key off..

at this point im at a complete loss.. i have 43 lbs of pressure at idle for fuel pressure.. and is great when im driving.. not like i run out of fuel... have more then enough at WOT etc..

was told that replacing the injectors would fix this.. was told that replacing the coolant temp would fix this.. basically was told all that i replaced would fix.. but still nothing..

it will idle like 8-900.. and then drop to like 7 randomly.. then go back up.. and be find for a bit..

seams to get worse when hot..

sounds like a clean or dead random miss..

any ideas?.. im working on getting a smoke tester to determine if it a vac leak.. info on tests would be great as i sure it can help others.. i have access to alot of stuff...

bill mcdonald

another vote for watching under the hood for spark shorts in darkness.

I had lots of these little bugs get me.

also check over spark connections. had one loose wire only slightly effect idle, a tiny bog on slow accel, and gone when I nailed it. turn AC on and my idle was a little more choppy.

lopezisgod

well noticed a new symptom...

WOT.. after a driver here and there.. car is at Operating temp..

downshifts and has no power what soever... stumbles and wont rev past like 26-2800

try again and same thing

cruise a bit more.. and punch it.. and car almost shuts down.. let off and stays running.. but touch throttle and wants to die completely.. stab the throttle couple more times..and she back to life.. wont stall when touching throttle but still has that WOT stumble..

(also my brake booster took a **** on me about 20 min later.. FML)

anyone help me out here?

im thinking a bad MAF... never changed it.. i try taping on it... while running.. and no change in any way..


???