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84 Crossfire - Rough Idle


Apr 29, 2010
1984 Gold Coupe
Hi All,
I am a newbie Vette owner and first time on this forum… I Just purchased an 84 crossfire Vette with 151k Kilometers. Good shape. Even though there’s a few gremlins I’m still very happy with the purchase.
Picked it up with a ‘rough idle and a miss’, therefore scheduled the usual tune up as the car was stored for the winter… Replaced
- Cap, rotor, ignition module (accel) and coil (accel)
- Replaced plugs (ac delco) and wires (accel)
- Changed oil and oil filter (K & N)
- Changed Air Filter (ac delco)
- Changed Fuel filter
- Changed Pcv

I thought that this major tune up would’ve cleared up the rough idle and miss but the car still idles rough… started the troubleshooting process and noticed that the EGR valve was no longer there and it was replaced with a block off plate.
I therefore went ahead and reinstalled a new EGR valve and installed the proper vacuum tubing to the EGR Solenoid and the TBI. That didn’t resolve the issue.

Since the car is over 26 years old I went hunting for vacuum leaks and didn’t find any (doesn’t mean there aren’t any) and started replacing the tubing that looked frail and damaged. That didn’t help.

Replaced the MAP sensor, no change.

Replaced an IAC with a new one as it broke when I was re-installing it on the TB following a thorough cleaning. No change. (when I set the ECM in diagnostic mode they both keep cycling open\close and won’t stop until I unplug the IAC --- Is this normal??)

Engine idled at 900 rpm in drive therefore, blocked off the idle air passages and Set the min air idle to 500rpm in drive. Readjusted timing from 10 to 6* with EST disconnected and set the TPS at .54. Reconnected everything back up and disconnected the battery to clear error code and started the engine …still a very rough idle at 800 rpm. Took it out for a drive to reset the IAC’s but it didn’t really improve anything. Monitored the Vacuum and the gauge indicated a rock solid 21hg.

The only thing remaining is to replace the Fuel pump and balance the TB’s (and now the heater core as it started leaking coolant in the passenger compartment yesterday)… which should be done over the next few days.

With ALDL cable and Winaldl ran a diagnostics and got the following Sensor data :
RPM 1000 - IAC 80, CT 49, MAP 241 (4.7V, 99.3 KPA), TPS .55v, Int 128, O2 136 (.601 v), Batt 24.9v, Knock Counter 12, BLM 128, Rich/Lean 162

Am I missing something ??
What else can I do to get this great car running fine again ??
Your opinions are very appreciated, thanks.:D:D
Maybe a compression check to find out where you're at cylinder to cylinder......
Andy Anderson
Think in terms of worn out Throttle body butterfly shaft bushings. You are probably dealing with the vacuum leak from this area. It's very common.
Hey BigJimZ16 - I've already verified if there was any play or looseness at the TB shafts and there was no play at all... I am leaning towards a possible vacuum leak at the TB lid even though a propane test revealed no leaks...

I will also do a compression test as suggested by navy2kcoupe, although the vacuum gauge and the old sparkplugs should've given me some type of indication if I had a problem with a respective cylinder.... or am I wrong ?


With ALDL cable and Winaldl ran a diagnostics and got the following Sensor data :
RPM 1000 - IAC 80, CT 49, MAP 241 (4.7V, 99.3 KPA), TPS .55v, Int 128, O2 136 (.601 v), Batt 24.9v, Knock Counter 12, BLM 128, Rich/Lean 162

I HOPE this data was recorded when the engine was not running (MAP 99.3kpa) Looks like it was as the IAC count is 80...a typical "park" count.
Yes, these readings were taken with the engine running but the car was in the park position...including the 24.9v identified by winaldl....
Any suggestions on things for me to work on ??
Check for any play in the distibutor, that could cause a wondering miss. Check the rocker arms, maybe one is too tight or loose.
Hey ex-x-fire,
I will verify the distributor as I was working on the car earlier and while I was checking the timing, the timing mark was quite shifty. Could that be an indication of a bad distributor ??
WAIT hold on...

If these measurements were recorded with the engine RUNNING...forget the distributor you have bigger problems.

First of all...24.9V is VERY wrong, running or not. This MUST be investigated. Get a good 'ol digital volt meter and measure the battery voltage directly, running vs. not running, or at least see what the gauge cluster voltage reading is. Somewhere around 14V is normal with engine running, 11-12V when not running.

As is a MAP reading of 99kpa...this is measuring full atmospheric pressure (right at 100kpa at sea level), and can ONLY happen either when the throttle is wide open, or if the engine is not running (thus not pulling a vacuum). If the computer "thinks" the MAP reading is this high at idle (or low engine loads) the fueling is going to be ALL wrong...probably horribly rich.

Chase these two down....something is very wrong with both of these readings.
geekinavette, I verified the voltage issue and measured the battery voltage directly its at 12.1 Volts and 14.3 Volts with engine running.

As for the MAP I verified its voltage at the harness between terminals a and b its at 4.9Volts.

I was also investigating the CTS and found both connector wires melted together the wire conducters were barely touching each other. The FSM says that the measurement between both wires should measure over 100ohms. At this point I get no reading. Is it possible that the ECM got fried ??
Not sure how else the ECM could report a "wrong" battery voltage unless its defective. Could be a grounding issue I suppose.

That MAP voltage of 4.9V....was that running or not running? (it matters a lot)

The 100 ohms is across the CTS leads while disconnected from the ECM. You can measure it connected, but that could effect the measurement. Even connected, the resistance can ONLY go down, it can not go up. If there is "no reading" then either the range of the meter is set too low (unlikely as 200 ohms is a common minimum setting...but check that) or the CTS is just bad, but either way no a bad ECM can not cause a "no reading" (or open circuit) when measuring CTS resistance.
A grounding issue where ??

As per the FSM, measured the MAP voltage with ignition on (engine not running) and the result was 4.9 V... what do u mean it matters a lot ??

As for the CTS, I got no reading from either the harness coming from the ECM or the CTS itself. Since the CTS pigtail is destroyed I'll replace both the pigtail and the Sensor. That should be one less thing to worry about.

How about the ECM, is there any way I can troubleshoot before swapping in a new part ??
An ECM grounding issue. Ground pins are:

White connector: 12, 13, 14
Black connector: 11, 15

Battery voltage to the ECM is on white connector pins 10 and 15. I would check continuity between all of those ground pins and the negative battery terminal.

It matters if the engine is running or not when measuring MAP voltage because it is measuring Manifold Absolute Pressure. If the engine is not running then it is pulling no vacuum, thus the pressure inside the manifold will be the same as outside the manifold and the MAP sensor will simply report normal atmospheric pressure. At sea level this is right around 100 Kpa (kilopascals), or close to 5V out of the MAP sensor. When the engine is running, say at idle, the manifold pressure is much lower than atmospheric pressure, since it is pulling a vacuum. With the engine idling the ECM should be reporting a MAP value of somewhere around the 20-30 Kpa range (if I remember correctly...either way it will be MUCH less than 100 Kpa), and should be much less than 5V (maybe around 2V or so, I'm not really sure).

This is why a big red flag came up when you said the ECM was reporting almost 100 Kpa WITH the engine running. The ONLY time you will see the MAP sensor report a very high value (near 100) is either under wide open throttle, or if the engine is not running. IF the ECM is reporting this very large MAP value with the engine running at idle, then you need to measure the MAP voltage WITH the engine running at idle. It should be much less than 5V at that point. If it is, yet the ECM is still reporting almost 5V, and the ECM ground pins check out (going back to the 24.9V battery voltage thing)...then yeah something is very flaky with the ECM.
Hi geekinavette,

Finally got around to getting under the dash and pull out the two ecm connectors...

Verified the white connector pins 12, 13 and 14. Pins 12 and 13 have continuity but pin 14 (map ground) is open. As for the Black connector pins 3, 11 and 15. Pins 3 and 15 have continuity but pin 11 (TPS and Coolant ground) is open.

Verified pins 10 and 15 on the white connector for battery voltage and both pins have 12.05 volts present.

I guess these open circuits explain a few things... What is the best way to solve this issue and do you think I should also concentrate on something else ???

No find and fix the missing grounds...these will cause you ALL kinds of trouble.

Just from memory...check ground wires at the rear of the driver's side cylinder head, and on the driver's side of the transmission bell housing. I don't recall which is which...but those two locations ring a bell. I'll try to find my FSM tomorrow and verify the locations of those two grounds.

Sorry for not replying yet...can't seem to find my FSM!

Hate it when I put stuff away somewhere safe then forget where I put it! :L
Hi Guys,

ex-x-fire, as you suggested I checked the Distributor and there is no play.

navy2Kcoupe, Did a compression test and the results are the following :


any other ideas ??

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