Engine EGR/EVR clarification

lopezelguapo

New Member
May 23, 2021
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Hello.

I have a 5.0 engine from a 1991 mustang swapped into an MGB. I used the OE fuel injection / intake / MAF with a Painless harness to connect a stock A9 ECU. The motor has AFR renegade heads and custom headers which fit the car (no smog pump or air injection system, no cats but it does have O2 sensors in each header after the collectors).

I've run into an intermittent lean condition which I am trying to diagnose and I've been reading up on the various possible culprits.

One possibility I've considered is that the ECU is determining that the EGR valve is open and pulling fuel, when the EGR valve is actually not being actuated (it's present and connected to the ECU via the harness but the vacuum supply is capped off). However, if I understand the system correctly, this shouldn't be the cause as the car has no EVR at all and there is nothing connected to that part of the harness. Am I correct that this will simply cause the ECU to bypass all EGR function entirely?

Thanks in advance...
 
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Mustang5L5

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Mod Dude
Feb 18, 2001
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Pull the codes on the ECU. You can verify if there is a code 33 which will disable the EGR in the ECU tune.

does the painless harness feature a diagnostic test port hookup?
 

lopezelguapo

New Member
May 23, 2021
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The lean condition seems random and pretty infrequent, like it showed up yesterday two times for maybe 15-30 seconds in the course of a 1 hour drive. At this point I am almost sure it is related to fuel delivery (specifically a bad fuel pump or a confused ECU not running the fuel pump often enough - fuel pressure is good at ~39 but the fueling system other than the fuel rail and injectors is kind of a roll-your-own solution I made when fitting the engine. When it happens I've been taking my foot off the gas and mostly creeping along at ~1500 rpm - it lasts for a few seconds and it will suddenly switch off - and I mean it feels like a literal switch being flipped, and then it's gone and the motor pulls hard as you would expect. I have a couple things to try out to confirm or eliminate that being the issue (like bypassing the ECU control of the fuel pump relay).

The painless harness does have the diagnostic test port - but unfortunately I am an idiot and I didn't notice that the STO and STI ports are not on the same terminal block with this harness. So the larger of the two (STO I think) I left accessible but the other one is wrapped up somewhere inside the nicely bundled and secured harness wrap I used when I wired the car. I am going to have to do some digging and disassembly to go and find it so I can pull the codes.

Anyway - it does sound like the EGR is likely not the issue here - the fact that it seems related to an electrical / electronic thing given the very clear on/off nature of it made me think that was one possible cause which would behave like that.

thanks!
 

lopezelguapo

New Member
May 23, 2021
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Los Angeles
Figured it out - The O2 sensors were the wrong kind (harness-grounded 4 wire instead of 3-wire internal ground). Running the car open-loop with the sensors just unplugged works great!

I have the whole donor car harness - or what is left of it - and I am going to pull some female pins from another connector to modify my harness to accept the 4th lead and ground out the 4th wires, and see if it runs properly in closed-loop mode. It is possible I suppose that even with the sensors grounded properly the headers could still create an issue with timing or insufficiently heating them, but I will try it and report back.
 

lopezelguapo

New Member
May 23, 2021
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Los Angeles
Just incase anyone else comes across this - adding the ground lead pin to my 3-wire (but 4-pin connector) O2 harness and grounding them to the chassis worked great. No codes and the motor is purring now. Just drove it for 2 hours with the 4-wire sensors and no issues at all.