My 86 gt vert has me scratching my head. It's jerking/bucking at low rpms. The idle is also all of a sudden is wanting to bog out but then comes back up continuously. It is speed density. I switched it to maf but had more problems. Black and blue smoke and when I let the clutch out rpms would fall then stall. Changed my iac and reset my tps. New plugs and wires also. I switched it back to speed density and now idle is sweeping trying to stall out. Smells terribly of gas and started this awful jerking. Burnt about 1/4 tank of gas in about 4 miles of driving. Could it be my egr? Or bad injectors? Any help would be awesome! Thanks in advance
Jerking and bucking! Bad gas smell!!
- Thread starter 86notched
- Start date
-
Sponsors (?)
Blown88GT
Founding Member
Have you measured the fuel pressure? With the car idling, remove the vacuum line from the fuel pressure regulator, & plug the line. Pressure should be 38-40 psi. With vacuum line connected, pressure should be about 30 psi (1 psi for every 2 inHG vacuum).
Doomstein
Member
Certainly could be your EGR, also make sure a vaccum line to the EGR isn't broken. If a vaccum line breaks all hell tends to break loose.
Haven't checked fuel pressure yet but good idea. I checked over all of my vac lines and all are good and not broken or cut. Also tried to pull some codes but whoever had this before did some electrical work and it won't even complete the test to flash codes. This all started because I failed inspection for low idle hydrocarbons. Replaced o2 sensors also. Inside of pipes are really black seems to be extremely rich..also did compression test when I had the smoke and all cylinders were at 150 with the exception of 2 holes were 135-140 which I think is ok?
Dump the codes: Codes may be present even if the Check Engine Light (CEL) isn't on.
See the "Surging Idle Checklist” for help with all your idle/stall problems. You can guess at the problem and throw parts at it, or you can use the checklist to help you find the problem quickly and inexpensively.
The quick and easy way to dump the codes is in there too, and all you need to do it is a paper clip! The first two posts contain all the fixes & updates. At last count there were 24 possible causes and fixes for surging idle/stall problems. I continue to update it as more people post fixes or ask questions.
See the "Surging Idle Checklist” for help with all your idle/stall problems. You can guess at the problem and throw parts at it, or you can use the checklist to help you find the problem quickly and inexpensively.
The quick and easy way to dump the codes is in there too, and all you need to do it is a paper clip! The first two posts contain all the fixes & updates. At last count there were 24 possible causes and fixes for surging idle/stall problems. I continue to update it as more people post fixes or ask questions.
Blown88GT
Founding Member
No CEL on an '86. When you went to MAF, did you remove vacuum line to MAP sensor & plug the vacuum line? When you went back to SD, did you reconnect the MAP sensor to vacuum?
txstang84
15 Year Member
My thinking is kinda in line with Blown88gt here...you said it was acting up before you did the swap to MAF, right? Did you happen to check your MAP sensor if it was connected or the vacuum line to it leaking? If the SD reads less than idle vacuum, it'll believe the engine is under higher load and enrich the mixture. I know you said you already checked for vacuum leaks, but have you hooked up a gage and checked actual manifold vacuum? The problems you're having sound much like vacuum signal issues.
On the MAF swap, did you use a known good computer and MAF sensor? Were all pinout changes verified good? Not tryin' to second guess you here...just running through a mental checklist...
EGR can cause some nasty issues, as mentioned above, check it for proper function...
When you swapped your plugs, were they all fouled, or just one or two?
The compression check is OK IMO, 135-140 is still within 10% of your highest reading of 150, so...probably nothing abnormal there and may've even been as simple as slight leak through the o-ring.
On the MAF swap, did you use a known good computer and MAF sensor? Were all pinout changes verified good? Not tryin' to second guess you here...just running through a mental checklist...
EGR can cause some nasty issues, as mentioned above, check it for proper function...
When you swapped your plugs, were they all fouled, or just one or two?
The compression check is OK IMO, 135-140 is still within 10% of your highest reading of 150, so...probably nothing abnormal there and may've even been as simple as slight leak through the o-ring.
Yea I double checked the map sensor and it was cool both maf and speed density. I'll get an accurate reading on my vac system tonight that's a good idea too. When I changed the plugs they all looked ok just old. If I remember correctly 2 of them were wet. I'm gonna run it and check the plugs again for gas. And yes the maf sensor and a9p computer are both in good working order. The swap went smoothly but took it back to sd to try and resolve my original problems first..seemed more powerful as sd rather than maf
As I said before , dump the codes. A bad MAP/Baro sensor will usually set code 22. When you convert to Mass Air, the MAP/Baro sensor gets vented to free air and does not connect to vacuum.
I'm having trouble even getting the codes. My wiring is all whacked out and it won't even complete the test. But I think I have the idle worked out..I tried a new map sensor with my vac line hooked up and no more jerking or bad idling!! Still have the smoke at the tailpipes tho. Thanks for all the input
You may have some signal ground problems, see the tech note below...
The computer pin 46 signal ground is a critical component: it provides ground for the
Map/Baro, TPS, ECT, EGR position sensor and ACT. Signal ground is used in many circuits that have analog inputs to isolate the electrical noise. It is always separate from power ground,
although both may have a common connection origination point. Signal ground usually
has some conditioning that reduces the electrical noise to prevent false readings
The black/white wire (pin 46) is signal ground for the computer. It provides a dedicated
ground for the EGR, Baro, ACT, ECT, & TPS sensors as well as the ground to put the
computer into self test mode. If this ground is bad, none of the sensors mentioned
will work properly. That will severely affect the car's performance. You will have hard starting, low power and drivability problems. What sometimes happens is that the test connector black/white wire gets jumpered to power which either burns up the wiring or burns the trace off the pc board inside the computer. That trace connects pins 46 to pins 40 & 60.
See http://www.stangnet.com/mustang-forums/749974-computer-issue.html#post7490537 for Joel5.0’s fix to the computer internal signal ground.
If the ground for the TPS goes bad, the TPS output voltage increases and the idle speed goes up.
Troubleshooting signal ground problems:
Note that all resistance tests must be done with power off. Measuring resistance
with a circuit powered on will give false readings and possibly damage the meter. That means disconnecting either the battery positive or negative cable since there is always some current draw due to the computer and radio circuits.
1.) With the power off, measure the resistance between the computer test ground
(black/white wire) on the self test connector and battery ground. You should see less than 2.0 ohms.
2.) MAP circuit: Check the resistance between the black/white wire on the MAP/BARO sensor and then the black/white wire on the EGR and the same wire on the TPS. It should be less than 1 ohms. Higher resistance than 1 ohms indicates a problem with the 10 pin connector or the splice
inside the main harness where the wire from the 10 pin connectors joins the rest of the black/white wire. Next check the resistance between the black/white wire and the negative battery cable. It should be less than 2.0 ohms.
3.) Engine mounted sensor circuit: Check the resistance between the black/white wire on the TPS and battery ground. It should be less than 2.0 ohms. Higher resistance than 2.0 ohms indicates a problem with the 10 pin connector or the splice inside the main harness where the wire from the 10 pin connectors joins the rest of the black/white wire.
Repeat the process for the ACT and ECT sensors. Your results should have the same specifications.
See the graphic for the location of the 10 pin connectors:
See the graphic for the 10 pin connector circuit layout.
The injector power pin is the VPWR pin in the black 10 pin connector.
The computer pin 46 signal ground is a critical component: it provides ground for the
Map/Baro, TPS, ECT, EGR position sensor and ACT. Signal ground is used in many circuits that have analog inputs to isolate the electrical noise. It is always separate from power ground,
although both may have a common connection origination point. Signal ground usually
has some conditioning that reduces the electrical noise to prevent false readings
The black/white wire (pin 46) is signal ground for the computer. It provides a dedicated
ground for the EGR, Baro, ACT, ECT, & TPS sensors as well as the ground to put the
computer into self test mode. If this ground is bad, none of the sensors mentioned
will work properly. That will severely affect the car's performance. You will have hard starting, low power and drivability problems. What sometimes happens is that the test connector black/white wire gets jumpered to power which either burns up the wiring or burns the trace off the pc board inside the computer. That trace connects pins 46 to pins 40 & 60.
See http://www.stangnet.com/mustang-forums/749974-computer-issue.html#post7490537 for Joel5.0’s fix to the computer internal signal ground.
If the ground for the TPS goes bad, the TPS output voltage increases and the idle speed goes up.
Troubleshooting signal ground problems:
Note that all resistance tests must be done with power off. Measuring resistance
with a circuit powered on will give false readings and possibly damage the meter. That means disconnecting either the battery positive or negative cable since there is always some current draw due to the computer and radio circuits.
1.) With the power off, measure the resistance between the computer test ground
(black/white wire) on the self test connector and battery ground. You should see less than 2.0 ohms.
2.) MAP circuit: Check the resistance between the black/white wire on the MAP/BARO sensor and then the black/white wire on the EGR and the same wire on the TPS. It should be less than 1 ohms. Higher resistance than 1 ohms indicates a problem with the 10 pin connector or the splice
inside the main harness where the wire from the 10 pin connectors joins the rest of the black/white wire. Next check the resistance between the black/white wire and the negative battery cable. It should be less than 2.0 ohms.
3.) Engine mounted sensor circuit: Check the resistance between the black/white wire on the TPS and battery ground. It should be less than 2.0 ohms. Higher resistance than 2.0 ohms indicates a problem with the 10 pin connector or the splice inside the main harness where the wire from the 10 pin connectors joins the rest of the black/white wire.
Repeat the process for the ACT and ECT sensors. Your results should have the same specifications.
See the graphic for the location of the 10 pin connectors:
See the graphic for the 10 pin connector circuit layout.
The injector power pin is the VPWR pin in the black 10 pin connector.
Similar threads
