Runs bad @ low RPM's

[olddsmer]

Hello Tuners
Just took out the Talon for a spin.Ran perfect for about 20 miles. Then while just cruising on the interstate it started to kind of sputter and hesitate. I pulled off and got it to a station and checked for boost leaks or some loose connection but didn't find any. I took off the neg cable for awhile to see if the computer would reset and that didn't work either. I decided to try to make it home. While cruising back, I just put it to the floor. When it hit around 4000 rpm it smoothed out and ran great. When I let off and got back to around 3k, it started to run like crap again. It boosts good, good oil pressure, good coolant level. Just wondering if I have a computer or O2 sensor going bad. It is dark now so I will have a better look tomorrow. Any suggestions will be appreciated.

Regards
Greg

 

[gofer]

Check for fuel at the rail, spark at the cylinders, and base timing is still spot on. Post your results and we'll continue troubleshooting it.

 

[olddsmer]

My fuel pressure gage reads 40 psi. I have spark from the plug wires. I haven't checked base timing yet but I would believe that it wouldn't run as smooth as it does at 4k and above if the timing was off. I will give it a look and see. I just can't see why it smooths out at higher rpm's and runs crappy at lower rpm's. It is not throwing a CEL either. Plugs and wires are about 2 weeks old. Gapped at .28.

Regards
Greg

 

[Calan]

What kind of chip is in it?

On the surface, it kind of sounds like it is open/closed loop related, rather than RPM based.

 

[olddsmer]

Just a standard ECU. I agree with you. That is why I think it may be related to
o2 sensor or computer going bad. I understand that an ISC motor going bad can affect the drivers in the ecu and eventually fry them. It just feels to me this is electronically related. Some kind of sensor or something feeding bad info to the control module.I am going to check the base timing to at least rule it out.
Like i stated before, when getting to the higher rpm ranges, the car smooths out and runs like butter. I got in this morning and the car started right up and seemed to be ok. Maybe as it heats up the problem will come back. I am getting ready to put an Eprom ecu in it. I ordered a new isc motor and o2 sensor
today. They are both old and probably need to be changed anyway as any maintenance item does. I will drive it tonight and see if it happens again.

Regards
Greg

 

[gofer]

My money is on the CTS going bad if it ran fine warming up, you can test it with a DMM instead of pitching the cash for one just to find out that wasn't the problem.

32*F - 5.9K ohms
68* - 3.5
104* - 2.7
176* - 0.3​

 

[Calan]

My money is on the CTS going bad if it ran fine warming up

Yep...or the O2 sensor.

If it runs fine while warming up and under moderate/heavy throttle (open loop conditions), then it definitely sounds like something isn't right once the ECU switches to closed loop.

 

[gofer]

In addition to the testing procedures I posted its also possible that the wiring at the back of your coolant temperature sensor broke loose. Check the integrity of the wires coming out the back of the sensor plug, they are known to become very brittle (especially on 1gs) and break. Heres the fix...

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[olddsmer]

Good info guys. I will test. I did pull the plugs today. Left to right, the first 2 plugs were kind of black and funky. I believe those are cylinders 4&3. The second two were nice and tan. Is it possible the coil pak or power transistor is going bad?

Regards
Greg

 

[gofer]

The coil packs, if you pay attention, run 1 & 4 or 2 & 3. It makes sense that 4 & 3 would be a bit "blacker" (resembling a rich condition) than 2 & 1 due to the fact that the fuel coming into the rail hits #4, then down the line. Typically if you have a cylinder "fail" because it goes lean its #1 because its at the end of the line, thats why back in the day guys would run their EGT gauge off the #1 runner on the exhaust manifold.

New plugs then for sure and test that CTS.

 

[olddsmer]

Good stuff Gofer. Also I noticed that 4 & 3 are served by the same coil module.
The coil pak is made of 2 modules. Each module services 2 plug wires. The 2 plugs that look kind of funky trace back to the same module. I wonder if that part of the coil pak is breaking down? My vacuum is a little lower than spec but I think that is due to the 264/272 cam profile. I know more agressive cams can kill some vacuum. I had some bpr6es ngk's in the garage so I did change them with no change in behavior. I should probably ohm out the CTS and that coil module and see where I am. I will probably just change that cts. It is probably the original one that came in the car.

Regards
Greg

 

[Calan]

The coil packs, if you pay attention, run 1 & 4 or 2 & 3.

Also I noticed that 4 & 3 are served by the same coil module. The coil pak is made of 2 modules. Each module services 2 plug wires. The 2 plugs that look kind of funky trace back to the same module. I wonder if that part of the coil pak is breaking down?

As Corey mentioned, the first coil pack should feed cylinders 4 & 1, and the second coil pack should feed cylinders 2 & 3. Our engines run a waste spark system, so technically you can swap 1 & 4 or 2 & 3, but you can't move a wire from one coil pack to the other.

Standing at the front of the car, the cylinders are numbered 4,3,2,1 from passenger side to driver's side. Standing at the driver side fender and looking at the coils, they are numbered 4,1,2,3 from the front of the car towards the firewall.

Sounds like you (or someone) has the plug wires crossed up.

 

[olddsmer]

Calan is right. I checked again in daylight and 4 and1 are on one module and 2 and 3 on the other. Cylinders 4 and 3 had the funky plugs. I have had the car for a while and always before the plugs were usually tan not sooty black. So something is not right. Going to head out and check the CTS.

Regards
Greg

 

[olddsmer]

The CTS ohmed out within spec. going to replace it anyway because it looks old. Should have my new o2 sensor in later today. If that doesn't work, I guess I'm down to a coil pack. I was thinking of the power transistor but have been told if it was bad, the car would not even start.
Suggestions are still welcome.


Regards
Greg

 

[gofer]

If the IPT was bad the car wouldn't start because you wouldn't be getting spark somewhere, the issue your having has nothing to do with spark. Did you happen to check the wiring going to the back of the CTS, it might not be a bad idea to ohm it out too and checking continuity. It is possible the front o2 sensor decided to lock up too,like you mentioned, but replacing the IPT or coils would be a waste of cash as it wouldn't fix the symptoms you're describing.

 

[olddsmer]

I will check the the wires to the CTs again. Replaced th o2 sensor, no luck.
So far I have changed wires and plugs, ohmed out the CTS, installed new o2 sensor. Did not replace the CTS yet. AutoZone ordered the wrong one. The old CTS ohmed to spec. I will check the wires again. My neighbor across the street races Mustangs and he says it sounds like ignition. I will do the CTS. If that doesn't work, I'm not sure what the next move is. The coil pak is the only other thing I can think of that may be causing the problem. I have a spare ecu that I could try if I need to. Oh well, it will get solved one way or another.

Regards
Greg

 

[gofer]

Its not the ignition, its not the ignition, its not the ignition. If it runs fine while its warming up and then starts idling like crap then its obvious the spark is there and its fine. Its possible the FIAV on the bottom of your throttle body isn't closing up after everything warms up, thus creating a bad vacuum leak... Its also possible you've got a straight up boost/vacuum leak, when was the last time you did a boost leak test?

Did I mention that its not the ignition? Your neighbor must own stock in Auto Zone if he wants you to keep replacing ignition parts, its not going to fix your issue.

 

[olddsmer]

Gofer. I believe you. I am checking the CTS wiring now. The only thing I got from auto zone is the CTS which is backordered. I already had plugs and wires bought for routine replacement. i did get an o2 sensor since mine was about 5 yrs old. Anyway I just checked the CTS wiring. With ignition on, car not running I should get 4.5 to 4.9 volts. I ain't getting it. It does not check out for continuity either. I guess I will keep moving away from the plug untill I get something. Does this circuit go through a fuse or relay anywhere?

Regards
Greg

 

[gofer]

Nope, the GRN/BLK (pin 1) wire goes to the ECU's sensor ground (pins 17 & 24) and the YEL/GRN (pin 2) wire goes to the ECU (pin 20).

1g ECU pin out

Its not the ignition.

 

[olddsmer]

I have pulled the harness off everything and pulled it up to where I can get to it.
I have opened the 2 wires clear to the large part of the loom over the intake manny. Turned the car on and still no voltage. I guess with everything unhooked I should still get voltage to those wires. I had to unhook the CTS,the sendor the o2 and some other single spade connector. I would think the ecu would send some voltage. I got nothin. ECU?

Regards
Greg

 

[Calan]

If I recall correctly, the CTS takes the ECU input to ground...so there wouldn't be any voltage going to it.

To test it, hook an ohm meter to it and measure the resistance cold and then submerge it in hot water and read the resistance again. I can't remember the numbers off the top of my head, but I believe it should drop to almost nothing when hot. It's been a while though... I'll see if I can find it in the FSM in the morning.

Sure it's not the ignition?

EDIT:

According to one diagram I've found, it looks like one of the CTS wires is grounded and the other is tied to a circuit with at least one pull-up resistor leading to 5V...so I'm guessing there should be some voltage at the sensor; but I have no idea how much.

 

[olddsmer]

The manual says with ignition on, car not running, you should have
4.5 to 4.9 v to the plug. I got nothing. Both wires go to ecu. One is ground and one is sensor input. I guess I have a ecu with a bad circuit. I'll put one of my spares in and see what happens.

Regards
Greg

 

[olddsmer]

Well I put in another ecu and still no voltage to the sensor. There has to be a break in the wire somewhere. Very discouraging. Don't really know what to do except try to run new wires.

Greg

 

[olddsmer]

Well, just for grins I went and bought a new multimeter and re checked everything. I have 4.7 v to the sensor plug and continuity. Put a new temp sensor in anyway. Started the car. Seems to idle fine. Runs rough once you start to drive it. If you jump on it, it smooths right out at about 4k. Builds boost fine. Runs good as long as you drive hard. At cruise you can feel a sputter.
So far I have checked for vacuum and boost leaks, put a new o2 sensor in and temp sensor. The plugs and wires are only about 3 weeks old. I took them out and put in the old stuff then changed back. Put in a new ECU that I had. I am out of ideas. My friend was a mitsu tech for 7 years at Performance Mitsubishi. He has always done my motor swaps. I guess I'll take it to him and see what he can do unless you guys have any more ideas. I appreciate the assistance so far.

Regards
Greg

 

[gofer]

Check base timing I guess, thats sweet you troubleshot it down to a faulty multimeter.