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 Post subject: 91 240 Surge- IAC and TPS code
PostPosted: Wed Jul 07, 2021 8:06 pm 
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Joined: Tue May 07, 2019 9:10 pm
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Help, what am I missing? 1991 240 NA. Car idles fine, but at 1500rpm it surges and won’t make it past 2000rpm. Getting an IAC (223) and TPS (234) code. What I’ve done:
- cleaned IAC
- replaced TPS
- cleaned throttle body
- cleaned maf
- clamp IAC and dies from idle or if rev up contiunes to surge. Seems like IAC working fine.
- unplug maf and the idle changes. Seems like maf is fine and no leaks.
- checked exhaust isn’t plugged
- no obvious vacuum leaks
- unplugged and replugged ECM

It seems like TPS to me… believe I have it set right to click when throttle begins to move. Checked voltage and it seems on/off. I read 1991 is on/off is that correct? If so what tells the computer to add gas then?
Car was running fine, parked it in a garage for a couple months and now it’s doing this.

Any recommendations before I pull harness or look at swapping ECM?


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 Post subject: Re: 91 240 Surge- IAC and TPS code
PostPosted: Sat Jul 10, 2021 8:07 pm 
Haha, I just built a W24 Octo-Turbo, now what?!
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Joined: Wed Jun 14, 2006 4:19 pm
Posts: 3124
Location: Lethbridge, AB
Is this car on LH 2.4 or 3.1? If 2.4 - the TPS is just a switch and not really a TPS as we would understand it. Can you hear the IAC click? Does the code keep coming back? So the TPS is new - so not that. Have you done a complete check of the MAF? Ignition working properly?

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 Post subject: Re: 91 240 Surge- IAC and TPS code
PostPosted: Sun Jul 11, 2021 8:24 am 
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Joined: Tue May 07, 2019 9:10 pm
Posts: 21
122_Canuck wrote:
Is this car on LH 2.4 or 3.1? If 2.4 - the TPS is just a switch and not really a TPS as we would understand it. Can you hear the IAC click? Does the code keep coming back? So the TPS is new - so not that. Have you done a complete check of the MAF? Ignition working properly?


Yes LH2.4 (82vin, 16maf #)
The IAC appears to be working as verified by the clamping method.
Yes the codes return when cleared.

What am I checking for with the MAF? It looked good inside when I cleaned it and it seems to be responding when I unplug/plug it in. It’s the next thing I’m looking at replacing. It seems like a control issue to me.

I think the ignition is working properly and I have a new cap and coil installed.


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 Post subject: Re: 91 240 Surge- IAC and TPS code
PostPosted: Thu Jul 15, 2021 4:19 pm 
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Joined: Tue May 07, 2019 9:10 pm
Posts: 21
New Maf didn’t help.


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 Post subject: Re: 91 240 Surge- IAC and TPS code
PostPosted: Fri Aug 06, 2021 6:16 pm 
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Joined: Tue May 07, 2019 9:10 pm
Posts: 21
Just replaced fuel regulator. No difference. Idles fine, surges at 2000rpm.

Fuel injectors are fine, unplugged each one and there was noticeable difference. I replace fuel filter when I got the car a year or two ago. Got another one just to confirm.

How do I check fuel pressure on these cars? I have the gauge kit, but there’s no good way to connect. Anyone have a specific part list to connect a guage?


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 Post subject: Re: 91 240 Surge- IAC and TPS code
PostPosted: Mon Aug 09, 2021 12:11 pm 
Haha, I just built a W24 Octo-Turbo, now what?!
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Joined: Wed Dec 21, 2005 8:40 pm
Posts: 3480
Location: Calgary, Ab
If you haven't changed it yet, change your O2 sensor. They typically get neglected and will cause a surge. They don't often cause the inability to rev past 2k RPM, though a damaged catalytic converter can. You may want to try dropping the exhaust between cat & downpipe and see if it revs cleanly afterwards - last-ish resort. Having a spare crank position sensor is also a good thing, so I would recommend grabbing one to try and/or buying a new one.

Throw a timing light on it to see what's happening to the ignition at surge/cut out. Check all plug wires with the timing light to be sure they're firing at that RPM, and check #1 against the timing marks to see if it's surging along with the engine or going full retard when your engine stops revving.

Are you still getting the codes after changing the TPS and MAF? If so, suspect your wiring harness or ECM. Neither are known to be trouble on '90s 240s but harnesses can get pinched or chafed, and ECMs can take on water from leaking windshields.

Overall it's best to troubleshoot each system from start to finish, rather than stopping & starting. Disconnect the plug from the ECM and check continuity & resistances across each sensor circuit from there.

As far as connecting to the fuel rail, it would depend on what your gauge kit looks like. I thought the later 2.4 cars had a schraeder valve on the rail? You may need to tap into the system after the filter, or in front of the rail, or to the schraeder valve. How full is your tank at the moment - any chance your intank pump could be dead and it's doing the classic starving/bucking thing?


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 Post subject: Re: 91 240 Surge- IAC and TPS code
PostPosted: Fri Aug 13, 2021 3:19 pm 
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Joined: Tue May 07, 2019 9:10 pm
Posts: 21
Already checked exhaust. Doubt it would be a oxygen sensor preventing rpm.

No schraeder valve, they were 93+ I think.

In the process of replacing fuel filter even though I did that a year or two ago. The line between pump and filter broke during replacement, waiting on part. Both pumps are working.

I don’t think it’s a timing issue, but checking that next. Then going through ecu connections and possibly replacing ecu.


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 Post subject: Re: 91 240 Surge- IAC and TPS code
PostPosted: Sun Sep 12, 2021 5:07 pm 
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Joined: Tue May 07, 2019 9:10 pm
Posts: 21
Finally got a fuel pressure gauge on. For anyone interested M14x1.5 kind of seems like the right thread, I do NOT recommend leaving that on for long term. Short term testing it seemed to seal okay. Ended up using a brake fitting block and using tap and die set to make things work.

Anyways, ~35psi pressure. Pressure was consistent at idle, increased rpm and when surging. When the regulator vacuum was disconnected the pressure increased to around 45psi.I’m not sure what pressure is supposed to be, but no pressure drop indicates to me that it’s not running out of gas.

Checked timing - indicator is difficult to get a good reading. However timing is decreasing with rpm. With fast throttle the timing sharply decreases. Going to check crank position sensor for crud or damage before trying to replace it.

After that I’m out of ideas. See if I can find another ECU I guess?


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 Post subject: Re: 91 240 Surge- IAC and TPS code
PostPosted: Fri Sep 24, 2021 8:16 pm 
Haha, I just built a W24 Octo-Turbo, now what?!
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Joined: Wed Jun 14, 2006 4:19 pm
Posts: 3124
Location: Lethbridge, AB
3 bar is the normal operating pressure - so around 45 psi is fine. The FPR vac reference will pull about 10 psi off the rail pressure at idle. At 2000 RPM you would not expect to see a fuel pressure drop. 20 inHg is about 10 psi - so idle. At 2000 RPM it should be at rail pressure.

Timing should advance with RPM not retard. Weird.

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 Post subject: Re: 91 240 Surge- IAC and TPS code
PostPosted: Thu Oct 14, 2021 7:26 pm 
Bone stock

Joined: Tue May 07, 2019 9:10 pm
Posts: 21
Tried a new crank position sensor. No luck.
Verified getting throttle position to c7 on ezk. Tried to trace it on board to t6000 transistor, is that correct? Had high resistance. Tried to re-solder, but no difference.
No corrosion or wiring issues found.

Throttle position sensor code went away? Still have the IAC code. Iac seems to function fine, but shouldn’t create the surging.

Timing on these are silly, maybe I’m use to Fords. Timing seems okay. I was mistaken before, timing advances until it hits the surging when the timing advances and retards matching the surge. Tried to even adjust distributor to see if it made any difference.

Doesn’t seem like it should be this hard… any suggestion before I try new computers?
Losing patients on this thing.


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 Post subject: Re: 91 240 Surge- IAC and TPS code
PostPosted: Fri Oct 15, 2021 9:37 am 
Bone stock

Joined: Tue May 07, 2019 9:10 pm
Posts: 21
Want to buy or borrow
Ezk 0227400169
Or
Ecu 0280000561


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 Post subject: Re: 91 240 Surge- IAC and TPS code
PostPosted: Mon Oct 18, 2021 10:57 am 
Haha, I just built a W24 Octo-Turbo, now what?!
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Joined: Wed Dec 21, 2005 8:40 pm
Posts: 3480
Location: Calgary, Ab
The distributor has nothing to do with timing in this case. It only points the spark at the right plug. Do you think the advance/retard "matching the surge" is just due to speeding & slowing RPM, or is it retarding causing the engine to slow, advancing causing the engine to speed, etc? If the knock sensor is misinterpreting piston slap or wristpin noise at a certain RPM as knock, it could be trying to retard the spark which might drop it out of the RPM range where it thinks there's knock.

Or it could be something else. It would sure be nice if LH2.4 had a computer interface... "tell me what you want!!!"


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