1998 Gt To Cobra Swap "please Help"

Welder7018

Member
Sep 27, 2016
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Just traded for this partially complete project which is a 98 gt with what is suppose to be a 01-04 4 valve engine. But I'm more an old school guy and don't know exactly where to start working out the bugs, that I think are most likely wiring, vacuum, and pcm issues. So major issues are electric fan not running (I think because the 4v has a sensor for the engine man. system that the pcm for the gt doesn't read). Next issue is it doesn't idle well ( i think could be vacuum and wiring issues prob. both but also pcm as well) also doesn't have egr (is delete). Third for now seems to have low end power (no miss) but bust out and comes back at higher RPM's (I'm not sure if this engine has IMRC but if so could be the issue). Also there is a sensor on the fuel rail (front driver side) that is not hooked up electrically, has a vacuum hose from back of upper intake (I'm sure this could cause problems is it the IAC and if so where might the wiring be IDK). I would appreciate any input, may be able to see some of this from pic but if need be I'll take more. Should I start with a PCM and if so off of what year, model, aftermarket again IDK like I said OLD SCHOOL new school me PLEASE!
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IMO your FIRST purchase should be an ODB2 scanner that is capable of reading operational PID's. This likely would make short work out of some of your problems. For other problems knowing the PID data will save tons of trouble shooting time as well as avoiding changing perfectly good parts.

Consider starting a relationship with a tuner that you trust. Pick a tuner/ODB2 scanner that your tuner uses.

Note, The PCM currently in the car is capable of running the DOHC motor with the correct modifications and tune.

Also. There are two versions of the IAC valve. One with a black vent and one without. The 98 model year Mustang should have a black vented version. Which IAC is currently installed?

Here's some more information on how to trouble shoot idle/IAC issues.
http://www.stangnet.com/mustang-forums/resources/troubleshoot-iac-idle-problems-1996-2004.13/
 
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IMO your FIRST purchase should be an ODB2 scanner that is capable of reading operational PID's. This likely would make short work out of some of your problems. For other problems knowing the PID data will save tons of trouble shooting time as well as avoiding changing perfectly good parts.

Consider starting a relationship with a tuner that you trust. Pick a tuner/ODB2 scanner that your tuner uses.

Note, The PCM currently in the car is capable of running the DOHC motor with the correct modifications and tune.

Also. There are two versions of the IAC valve. One with a black vent and one without. The 98 model year Mustang should have a black vented version. Which IAC is currently installed?

Here's some more information on how to trouble shoot idle/IAC issues.
http://www.stangnet.com/mustang-forums/resources/troubleshoot-iac-idle-problems-1996-2004.13/

IMO your FIRST purchase should be an ODB2 scanner that is capable of reading operational PID's. This likely would make short work out of some of your problems. For other problems knowing the PID data will save tons of trouble shooting time as well as avoiding changing perfectly good parts.

Consider starting a relationship with a tuner that you trust. Pick a tuner/ODB2 scanner that your tuner uses.

Note, The PCM currently in the car is capable of running the DOHC motor with the correct modifications and tune.

Also. There are two versions of the IAC valve. One with a black vent and one without. The 98 model year Mustang should have a black vented version. Which IAC is currently installed?

Here's some more information on how to trouble shoot idle/IAC issues.
http://www.stangnet.com/mustang-forums/resources/troubleshoot-iac-idle-problems-1996-2004.13/

I do have a scan tool (is it capable of reading "operational" PID's not sure). As for a Tuner I don't know anyone and not sure where to look (small town). The first issue I need to work out is the fan not running, also the temp gauge is pegged. It looks like in the manual that these could be reversed (opposite sides in similar locations) from the SOHC to the DOHC, is it possible that they are hooked up wrong. Or if not is the sensor on DOHC not being read correctly by the PCM (are these sensors different one engine to the other). As for the IAC its not hooked up and can't find the correct wiring. I'm adding pics of things left unhooked (including IAC) I'm not sure what to do with. PS can you tell what year range this engine is by looking at the pics. Thanks in advance for your time and help it is greatly appreciated.
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from the picture it appears to me that the IAC is electrically connected. However it's the model without the black vent. Normally the 98 and older use a black vented model to provide starting air.

The EVAP purge system appears to be open to air. Which will cause problems each time the PCM calls for EVAP purge.

The EGR system appears totally disconnected. The DPFE is wide open. Expect problems unless EGR is disabled in the PCM.

Regarding the temperature sensors. For the 1996-1998 model year there are two. One for the PCM the other for the dash. The sensors are different and are NOT compatible to each other. Here's a case where asking the PCM what the PCM "thinks" the ECT temperature is would be a big help.

Also, the DOHC motor is very bad about trapping air inside the cooling jacket. If the ECT sensor is in contact with air instead of coolant it will read low. This could create a situation where the motor is hot but the PCM does not call for fan.

However based upon the dash gauge being pegged, I think that the temperature sensors being reversed or incorrect is more likely.

The other problem I see is the fuel system. The 1996-1998 model year normally has a return fuel system. The 1999+ has a return-less system. I can see the FRPS of the return-less system. However the sensor isn't hooked up.

However it appears to me that the fuel return line has been plugged which is not good for a return based fuel system.

WHICH PCM is performing engine management duties? If it's the original 98 PCM then it seems to me that the motor set up needs to be closer to what the 98 PCM is expecting. That means a return based fuel system.

Sounds like you could use a Ford service manual and a good set of wiring diagrams. If interested in getting a copy for yourself I maybe able to help. PM if interested.
 
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