 
     |
| 1986 GT stumbles badly off idle (Page 4/4) |
|
Patrick
|
MAY 10, 06:40 PM
|
|
| quote | Originally posted by balaseca:
I need to get me a cable.
|
|
Yes, for sure.
I've seen it so often here in the forum where people throw all kinds of money at unnecessary parts, instead of spending less money on a cable and figuring out the actual problem. In a recent thread Here, all sorts of time, money and effort was wasted on chasing the reason for a "high idle" when it turned out to be mostly a wonky tachometer.  (It was also discovered that there was no signal going from the MAP sensor to the ECU.) A scanning program (with the required cable) revealed the situation immediately![This message has been edited by Patrick (edited 05-11-2025).]
|
|
|
balaseca
|
MAY 11, 06:11 AM
|
|
|
To be honest, most of my wrenching experience is with far older vehicles, so I have to remind myself that datalogging is something I have access to. Thanks again, and I'll report back once I have a cable and data to present.
|
|
|
|
armos
|
MAY 11, 07:36 AM
|
|
WinALDL is how I discovered that my IAT sensor needed to be cleaned. Until I did that, it was reading very hot temperatures (over 200F if I remember right). There were a few things going on with the car at that point so I'm not sure, but I don't think the IAT caused any severe symptoms for me. It was the opposite fault from you though - with the IAT disconnected, you'd be getting faulty *low* temperature readings, not high temp.
Did you get a Check Engine code when the IAT was disconnected? If so, then the ECM was probably ignoring the reading and using some default value.
If you did not get a Check Engine code, then I'd check if the bulb is working (you should see it light up for a second when you turn the key on).
| quote | | Pressure at the schrader valve is 41 PSI when reved. 38 PSI at idle, 41 PSI with key on and motor off. There is some leak down after the key is turned off. |
|
If the leak down is a loss of 6psi in 10 minutes, that doesn't sound like a big deal to me. If it's dropping *to* 6psi in 10 minutes, that's a lot worse.
38psi when idling, 41psi when revved sounds like the FPR is behaving normally. It should maintain a constant pressure with respect to the air pressure in the manifold. Higher vacuum = less air pressure = less fuel pressure. This is so that the injectors will have a consistent flow rate so the ECM always gets the same amount of fuel for the same opening time.
Can you trigger a stumble while still being able to watch the pressure gauge? Is the pressure staying up when it stumbles?
MAP:
Our MAP sensors are 1-bar. Not so easy to know what the correct current reading should be, though.
I've heard of the idea of electrically disconnecting the MAP and see if it runs better that way. Unplugging it should cause an error code and the ECM to ignore the readings, so if it runs better that way then it would imply the readings from MAP are faulty.
| quote | | I'm about to make a seperate thread about my non-functioning VSS too. |
|
If I'm not mistaken, a nonfunctioning VSS will make the car drive badly, at least between stopping and starting. Not sure if it matters while already at speed though, and I don't see how it would impact revving at idle.
|
|
|
|
Patrick
|
MAY 11, 03:44 PM
|
|
| quote | Originally posted by armos:
If the leak down is a loss of 6psi in 10 minutes, that doesn't sound like a big deal to me. If it's dropping *to* 6psi in 10 minutes, that's a lot worse.
|
|
You bring up an excellent point! I read the OP's post as meaning it had dropped to 6 PSI and responded accordingly. I was probably mistaken here. Glad you caught that.
| quote | Originally posted by balaseca:
Well, I did the pressure test. 41 PSI with key on and engine off. After 10 mins, it had dropped 6 PSI.
|
|
|
|
|
|
balaseca
|
MAY 11, 07:29 PM
|
|
Lol, It only dropped 6 PSI, not *to* 6 PSI.
VSS issue has been solved
I did trigger the stumble and watch the gauge, stayed at 41 PSI.
I've tried unplugging the MAP, but the car wont stay running if I do...
I guess I need to get that ALDL cable and look at ignition related things as well as the MAP.[This message has been edited by balaseca (edited 05-11-2025).]
|
|
|
|