Stumble under ANY load??

[DrYeRLiNt]

I have a 98 v6 mustang that has been v8 swapped.

The car starts and idles perfectly with no problems at this point. If you rev it in neutral slowly it's fine, but if you give it some major throttle (>40-50% i'd say) it will miss a bit. This basically only occurs below 3000 rpms, after that it does not seem to miss.

Then, if you put it in gear (1, 2, D, R... though R will spin the tires slightly in our gravel driveway then go back to the same problem) and just let it go on it's own it'll stumble slightly but slowly pick up. IF you give it ANY throttle (5%-100%), it will stumble and not get out of it's own way.

I have never had it out on the road for fear of it stalling or not making it up a hill on me. Since our driveway is relatively short, I've not been able to even get it above 1200-1500 rpms in gear.



I've changed or checked- fuel filter, fresh gas (mixed with what was in there and some lucas fuel stuff), spark plugs, TPS has been checked, new EGR solenoid, changed MAF sensor, new coil pack, etc

Fuel pressure has been measured at a steady 30 psi at idle up to 40 psi at WOT.
The OBD2 scanner throws codes for the transmission and for O2 sensors.

I'm completely lost on this thing. Can anyone either suggest to me a good (affordable too) modular mechanic near Knoxville, TN or give me some insight as to what might be my problem?

 

[Maxpowers]

Please give a explanation of what all you changed to go from the v6 to the v8. This will help to diagnose what you may have missed.

 

[trinity_gt]

I was tempted to say check the fuel pressure but since you already did that, here's something you might want to check: What PCM was used, from what year and was traction control installed on that car?

It almost sounds like it's acting like the PCM is stepping in to mitigate wheelspin. If the installation of the new PCM and harness didn't capture everything needed for traction control as well, it cause the system to misbehave.

 

[DrYeRLiNt]

In the swap, I used:

98 GT ECU
98 GT ECU harness
97 Tbird 4.6 2v
97 Tbird Engine/trans Harness
97 Tbird 4R70W

The PATS system has been disabled, so the car will start and idle perfect.


I do not know if the car had traction control, but it has no power even if you slowly ease onto the gas pedal to prevent wheel slip from ever becoming an issue.

I have checked the TPS voltage, cleaned the IAC valve. Changed the injectors and spark plugs. I replaced a damaged coil pack (both have shown good continuity). I replaced the EGR solenoid.


It does has codes being thrown that show up on an OBD2 Scanner, I will update with a list of the exact codes when I get home. They all deal with O2 sensors and transmission things (shift solenoids).

Things that are definitely missing: overflow coolant tank. I have one on it's way that I purchased on ebay for ~$20. I know it has a sensor in it, but I doubt that the computer has any kind of limp mode associated with no signal from that sensor.

The spark plug wires had not been changed while I've owned it, but the car has had this stumble from when I bought it about 5 months ago, and I was led to believe they were changed shortly before I bought it.

It's literally like the timing is off or a poor/weak spark.


I did check the alternator with a multimeter, and it is putting out 12-13V DC and a considerable AC voltage as well. I know the rectifier in it (or one/several of the rectifiers since I believe they have about six) must be going bad, but could this really cause my problem?

 

[DrYeRLiNt]

In an attempt to be as thorough as possible with my explanation:

The car has the A/C lines disconnected. I also replaced the plastic intake because the one it had was leaking from a hairline crack. The car has a hydroboost setup from a GT mustang as well.

 

[nyuk98gt]

DrYeRLiNt:

I bet it is the plug wires. Before we get there, please confirm that the wires are on the correct posts on each coil pack. If these are incorrect then a cylinder is getting spark when it does not want it and not getting spark when it needs it.

Pass *** Driver
engine
6-3 *** 7-2
1-5 *** 4-8
front bumper

Pass Driv
firewall
4 - 8
3 - 7
2 - 6
1 - 5
radiator

I hope the sketch is clear.

Measure the resistance of each wire and measure the length. Then calculate the ohms/inch or ohms/foot. If one wire is high in resistance then it is faulty.

Next, use an old plug to confirm that the boot fits correctly and the wire clips onto the top of the plug. You may need to slide the boot up/down the wire. Move it about 1/8" at a time and don't bend the wire or it (the inner conductor) will break. Use a small dab of dielectric grease in the boot to keep moisture out, too.

HTH,

Chris

 

[DrYeRLiNt]

I didn't have an old plug to check if they snap together well but I would imagine a poor connection would give me a sporadic miss but its consistent.

The wires run to the right plug, and the wire resistance was all within 30 ohms/inch of each other. I figure that's within a decent tolerance level.


Any other suggestions?

 

[midnightdriver]

How are your spark plug gaps?

Did you re-gap them to make sure they were spec or did you just believe from the box that they were at the right spec?

Also what kind of plugs did you use?

 

[DrYeRLiNt]

I used the el cheapo autolite plugs. They said do not gap so I left them alone.

 

[DrYeRLiNt]

The car had the problem before I changed the plugs. Is it possibly a discrepancy between the computer and tbird harness?

 

[trinity_gt]

The car had the problem before I changed the plugs. Is it possibly a discrepancy between the computer and tbird harness?

This is what I was getting at with the traction control question. Not implying that you were spinning the tires but rather that because of some wiring, sensor or other issue, the PCM thinks there's wheel spin.

Wouldn't really explain the issues you see in neutral though...

 

[DrYeRLiNt]

Yeah. It's not a traction control issue. It just seems like the car has no power due to either fuel spark or timing.


I don't know what to do at this point.

 

[trinity_gt]

Yeah. It's not a traction control issue. It just seems like the car has no power due to either fuel spark or timing.


I don't know what to do at this point.

Traction control works by cutting fuel and/or timing...

 

[DrYeRLiNt]

I should have elaborated a little more. Everything I can find online indicates that traction control was not even optional on any trim of mustang in 1998.

 

[nyuk98gt]

DrYeRLiNt:

Correct: '98's did not have traction control in any form. ABS was an option.

You can check TCCoA.com (T-Bird and Cougar Club of America site) to see if some one there has used a Mustang PCM in their car. It should work as long as it is flashed for what the car has (stick or auto, the '98 fuel pump is different from the earlier model years, etc.). Note that an auto PCM is physically different than the stick PCM (for '98): you can run a stick with an auto PCM but you can't run an auto with a stick PCM.

The issue sounds spark related to me so you might check the '97 T-Bird firing order. 1998 was an in-between year for the Mustang and that model year has some unique differences from earlier models. Can you datalog anything? Data is like gold when trying to see what is going on under the hood.

Chris

 

[Mystic1350]

maybe try getting a tune...i did similar swap long time ago (mine was manual tranny tho) and a tune corrected all timing/computer issues.. jsut a thought.
at american muscle they can do mail order tune for $50, just a thought

 

[DrYeRLiNt]

I checked the firing order and it is identical between the mustang and t-bird.

One thing that must be mentioned... the computer in my car had to have the PATS disabled. Is it possible that there were other changes made as well during that process?



I have the option of purchasing a wideband 02 sensor, or a programmer. I can't afford both, so it's one or the other. The ones i'm looking at are the Zeitronix wideband 02 which is datalog capable, or the diablosport predator. Which would be better as far as diagnosing/fixing problems i'm encountering?

Also, while i'm asking... does anyone know if it's possible to remove the auto trans stuff from the computer with a predator? Eventually down the road, I'd like to swap a 5 speed into my car, but i'm not sure what would be required to remove that portion from the computer. Thanks.

 

[nyuk98gt]

DrYeRLiNt:

If the PCM was flashed just to disable PATS then that is all that was turned off. Was this done with a handheld (Predator, XCal, Sniper, etc.) or by Ford using their Star NGS device? If the latter then it is a safe bet that only PATS was turned off. If it was by handheld, you would have to confirm with your tuner about the parameters in the tune.

A handheld tuner that allows you to datalog is a huge benefit. You can add a wideband later, if you want, but I recommend the handheld. Call Diablo and confirm that the Predator can datalog (on-device or through-device to a laptop). I don't know anything about Diablo's stuff so I can help out much. My trusty XCal has been indispensible for tuning. Data is what you need to see to resolve engine issues. Otherwise, you will just throw parts at it and get frustrated.

Back to your original issue:

a) What condition is the TPS in? If you swapped it then it could have become damaged (it is quite delicate). This may be an issue.

b) Next, the Haynes manual has a nice write-up on checking the resistances on the primary and secondary coil circuits. Check those out. If you are losing spark then, obviouly, the engine will not perform.

Chris

 

[DrYeRLiNt]

I installed a wideband, and at idle the car is sitting around 13ish, but as you give it gas it climbs all the way to 18ish at WOT.

I've clearly got a fuel problem, but I'm not sure what all the possibilities could be.... especially since I'm reading 30 psi fuel pressure at idle and up to 40 psi at WOT. Is that low?

 

[Rickyll7]

If you are using a 98 gt ecu, you need to have the correct MAF and injectors for that ecu. What injectors and maf are you running? What transmission did the 98 GT autos have? Not sure if that would make a difference.

Your fuel pressure seems to be spot on.