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84 Crossfire - Rough Idle

joey9011

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Joined
Apr 29, 2010
Messages
19
Location
Montreal
Corvette
1984 Gold Coupe
Hi geekinavette,

I re-verified those two grounds and here is my dilemma...

I disconnected the connector from the map and took the readings from the ecm connector pin as well as from the map ground pin did the same for Coolant Sensor and TPS) and guess what continuity.

Now, when I plug the cable connectors back into the MAP,TPS and Coolant Sensor I don't get continuity (open circuit).... Is this normal ???

Thanks
Joey
Did you find/fix those two missing grounds?
 
Joined
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Messages
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Colorado Springs, CO USA
Corvette
84 Z51 auto R.I.P. 89 black roadster SOLD
Deal with one connection at a time...else you'll end up going in circles.

Let's worry about JUST the ECM connector, forget the actual sensor connectors. If those ground pins on the ECM connector do not actually connect to ground...that MUST be resolved.

What point are you using for ground itself? (for connecting the meter to)
 

joey9011

Member
Joined
Apr 29, 2010
Messages
19
Location
Montreal
Corvette
1984 Gold Coupe
Hi geekinavette,
Ok, The ground point that I use is the negative side of the battery.
I thought this was the best place... Should I splice into those ground wires from the ECM connectors and connect directly to ground ??
I really appreciate you're help.
Thanks
Joey

Deal with one connection at a time...else you'll end up going in circles.

Let's worry about JUST the ECM connector, forget the actual sensor connectors. If those ground pins on the ECM connector do not actually connect to ground...that MUST be resolved.

What point are you using for ground itself? (for connecting the meter to)
 
Joined
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Messages
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84 Z51 auto R.I.P. 89 black roadster SOLD
Yeah neg battery post is good.

No I wouldn't do any splicing. This pins are supposed to be grounded somewhere...need to find out why they are not.

Did you check those two ground points...rear of pass. side cylinder head and pass. side of transmission bell housing? If memory serves there are two ground wires on the cylinder head and one on the bell housing, or vice-versa. (sure wish I could find my FSM!)
 

joey9011

Member
Joined
Apr 29, 2010
Messages
19
Location
Montreal
Corvette
1984 Gold Coupe
Yes, I did verify the ground points... behind the cylinder head (on the drivers side) the one on the transmission housing and it all checks out.

The FSM doesn't really specify where those two pins are grounded to....

I'll continue searching the fsm as well, maybe I missed something.
thanks
joey
 
Joined
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Messages
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Location
Colorado Springs, CO USA
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84 Z51 auto R.I.P. 89 black roadster SOLD
The info is in the FSM...its just not exactly straightforward to derive where the actual ground points are.

I need to find that dang thing...
 

joey9011

Member
Joined
Apr 29, 2010
Messages
19
Location
Montreal
Corvette
1984 Gold Coupe
Hi geekinavette,

I'm really scratching my head on this one...

I've checked and re-checked the FSM as well as researched the issue for many hours on the net and the result is always the same, Pin 11 (from the black connector) does not terminate at any ground location ...
It starts at the ECM pin 11 (Black wire), its spliced and goes to the TPS and CTS.

The same is also true for pin 14 (from the white ECM connector), the purple wire is spliced and goes from the MAP to the fuel pump relay... not to any ground location.

What am I missing ???

Thanks
Joey9011
 
Joined
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Messages
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Location
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84 Z51 auto R.I.P. 89 black roadster SOLD
I think that makes sense....

Refer to wiring diagrams below, Black connector is C452, White is C451.

It seems to be that White-14 (MAP gnd) gets its ground connection from within the ECM, so with the connector unplugged it makes sense that you would not find continuity from that pin on the connector to ground.

As for Black-11. According to the diagrams this is the same ground as the EST Module Wiring Shield.

What's interesting is that neither diagram shows an actual external ground connection from this pin (Black-11), so it may very well be that it also gets its ground connection from within the ECM (via the "System Ground" pins, White 12 and 13).

This is not surprising. It is done most likely to avoid noise-inducing ground loops.

Here's what I would do...Connect the ECM back up and check for a ground connection directly at the sensor(s)...MAP purple wire, TPS black wire, CTS black wire (if the CTS is not already a 2-wire unit, replace it with one).

Check that out and we'll go from there.


ECM_sensors.jpg

ECM_ignition_CE%20lamp.jpg
 

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joey9011

Member
Joined
Apr 29, 2010
Messages
19
Location
Montreal
Corvette
1984 Gold Coupe
Hi geekinavette,

I finally installed a new ECM since the FSM was continuously pointing towards its replacement. I then verified the MAP, CTS and TPS grounds as you suggested..... with good results, continuity on each circuit.

I started the vehicle and finally a much smoother idle (although a little higher rpm, its much smoother).

Now, I have to resolve a new problem... hard start when its warm and finally do a throttle balancing...

Thanks
joey9011
 
Joined
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Messages
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Location
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84 Z51 auto R.I.P. 89 black roadster SOLD
Cool you're getting there!

Are you getting a Check Engine Light?

One quick thing to check regarding the hot start issue...run it up to temp with the air cleaner lid removed, make sure the fuel pattern out of the injectors is a nice solid "cone" pattern. Then shut it off and check for any fuel dripping from the injectors, there should be none. If the spray pattern looks "ragged" or there is any fuel dripping after shutting the engine off then you either have a leaking O-ring or the injectors need cleaning.

DO NOT ever look straight down into the throttle bodies while engine is running! Even a minor "backfire" will cause very unpleasant things to your face. Always monitor fuel spray from a safe distance.
 

joey9011

Member
Joined
Apr 29, 2010
Messages
19
Location
Montreal
Corvette
1984 Gold Coupe
Hi geekinavette,

No check engine light...

After replacing the ECM, started the engine and let it come up to normal operating temperature. It felt good and it was idling a little higher than normal at 1100 rpm.

I then unplugged the IAC's as well as the EST , plugged up the min air passages and set the min air idle at 500-550 in drive (450 car almost died). It's now running solid at 700 rpm in drive and 800 rpm in park.

I verified the fuel pattern yesterday, I get a nice cone pattern with no dripping.

Could the hardstart be caused by throttle bodies not synchronized or because of the higher idle ??

Thanks
Joey

Cool you're getting there!

Are you getting a Check Engine Light?

One quick thing to check regarding the hot start issue...run it up to temp with the air cleaner lid removed, make sure the fuel pattern out of the injectors is a nice solid "cone" pattern. Then shut it off and check for any fuel dripping from the injectors, there should be none. If the spray pattern looks "ragged" or there is any fuel dripping after shutting the engine off then you either have a leaking O-ring or the injectors need cleaning.

DO NOT ever look straight down into the throttle bodies while engine is running! Even a minor "backfire" will cause very unpleasant things to your face. Always monitor fuel spray from a safe distance.
 
Joined
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Messages
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Location
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Corvette
84 Z51 auto R.I.P. 89 black roadster SOLD
Sounds like one or both of the IAC valves may be leaking. Pull the IAC motors and make sure the pintle(s) are clean. Same with the pintle seat in the TB housing.

Also check for play where the throttle shaft goes through the TB housing. If there's any noticeable play you might want to consider having bushings installed as this can cause much misery with getting the TBs balanced.
 

joey9011

Member
Joined
Apr 29, 2010
Messages
19
Location
Montreal
Corvette
1984 Gold Coupe
Sounds like one or both of the IAC valves may be leaking. Pull the IAC motors and make sure the pintle(s) are clean. Same with the pintle seat in the TB housing.

Also check for play where the throttle shaft goes through the TB housing. If there's any noticeable play you might want to consider having bushings installed as this can cause much misery with getting the TBs balanced.


Hi geekinavette,

Removed both IAC's and cleaned each pintle and pintle seat... not very dirty as it was cleaned not long ago. Verified the pintle distance and they were both under 1&1/8 of an inch.

Checked the throttle shaft for play and there isn't any. I also double checked the injectors if there was a drip after shutting off the engine and there is no drip.

Anyways, it's still a little difficult to start the engine when warm.... I guess I'm ready for a TB balancing.... what do you think ??

Thanks
Joey
 
Joined
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Messages
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Corvette
84 Z51 auto R.I.P. 89 black roadster SOLD
How many miles are on this engine? I have never seen a crossfire TB that didn't have worn throttle shaft holes. The tension on the returns springs might be "masking" it.

Anyway, I doubt this is causing your hot start issue...but still something worth checking out, in addition to balancing. There's no downside to doing it.

It still sounds like there's a vacuum leak somewhere, as these things are notorious for. Common culprits are the intake plenum lid, EGR valve and of course all of the 20+ year old rubber vacuum lines.
 

joey9011

Member
Joined
Apr 29, 2010
Messages
19
Location
Montreal
Corvette
1984 Gold Coupe
Hi geekinavette,

Sorry for the delay...

There's 151k miles on this engine.

I've checked for vacuum leaks around the engine bay and have found nothing. I did the propane and squirting carb fluid all over the place and no leak. I also did the same test around the throttle shafts and nothing there either.

If there was a vacuum leak the engine would run with the throttle blades fully closed, right ??.... mine does not start or dies when I close the throttle blades completely.

I've verified the EGR valve (its new) and its fine... there's no vacuum at idle. As for the vacuum lines, I've replaced all of them in the engine bay... I may have a culprit within the dash area (heater) though...

Built the Manometer (today) and did the throttle balance... I noticed a smoother idle. I'll drive it on the road tomorrow morning and come back with the results.

Thanks
Joey
 
Joined
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Messages
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Location
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84 Z51 auto R.I.P. 89 black roadster SOLD
If there was a vacuum leak the engine would run with the throttle blades fully closed, right ??.... mine does not start or dies when I close the throttle blades completely.

No...because although the engine can get air (via the IAC ports also), it still needs fuel, which only has one path...past the throttle blades.
 

joey9011

Member
Joined
Apr 29, 2010
Messages
19
Location
Montreal
Corvette
1984 Gold Coupe
No...because although the engine can get air (via the IAC ports also), it still needs fuel, which only has one path...past the throttle blades.


Hi geekinavette,
Ok, I'll check for a vacuum leak once again....
Joey
 

joey9011

Member
Joined
Apr 29, 2010
Messages
19
Location
Montreal
Corvette
1984 Gold Coupe
Hi geekinavette,

Following the last post... I wanted to remove any doubts with respect to a vacuum leak from the toplid, I had the gaskets on hand and decided to replace the gasket. At the same time re-verified the throttle bodies and they already have bushings in them (I guess the last owner had them redone).

Dang, I still have that rough idle...
What am I missing ?? I've been through the owners manual at least 20 times looking for something I may be missing or doing wrong.

Any thoughts ??

Thanks
Joey

No...because although the engine can get air (via the IAC ports also), it still needs fuel, which only has one path...past the throttle blades.
 

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