1998 T5 surging at idle, code 1505 but replacement IAC encounters same problem

[JoebobNLacey]

I've searched the other posts and haven't found an easy solution yet. One older post was close in description of symptoms but never offered a solution.

1998 T5 V70 manual with a bunch of aftermarket high performance upgrades from prior owner. Haven't been under the hood for any work since last summer so problem developed on its own. Started with just occasional surges at idle. Eventually became more routine, and finally throws 1505 code, Idle Air Controller. Behavior: When fully warmed up, idle stays at 1300rpm, surges up to 1500rpm, drops back down within a few seconds, repeats. Never stalls. Seems to drive fine under load, on the highway, etc.
Removed IAC, cleaned it, reinstalled, reset codes. Within 30 minutes of operation, same problem. Took spare IAC from junkyard, verified same part number, cleaned it, reset codes, same problem within 30 minutes. I can't believe that I have two bad IACs, which are not known for having problems. Something else seems to be causing this, but not tripping a separate code. I know vacuum leaks are always a potential cause, but I've already replaced a bunch of lines and am not seeing anything. Don't have a smoke machine at my immediate disposal but can get one (90 minute drive away).
Engine seems to smell unusually hot. Generic OBII scanner claimed engine temp was something like 230F which seems way too hot, yet dashboard gaugh is perfectly at 3 o-clock, (normal). Electric cooling fan seems to run as expected. Exhaust smells off too. Next target is probably going to be engine coolant sensor based upon my research, but would love suggestions from any experts.

[abscate]

Unmetered air will mess up your idle , and if it runs well at revs, that also points to unmetered air.

It’s a drag, but you have to remove an inspect each vacuum hose on the engine, including the hidden ones. If they are 28 years old, they are past their ptime

[Sveedy]

I picked up a smoke machine for well under a $100 ( Autool SDT 60 - made in China, but REALLY well made ).
Kind of invaluable when working on these old rigs with old hoses.

[ZionXIX]

Ive been using this one for more than a years with excellent results.

Screenshot_20260217_125417_Amazon Shopping.jpg (80.81 KiB) Viewed 194 times

[MoVolvos]

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WOW, what a price, not sure which to choose AUTOOL or Solary? UPDATE: AUTOOL Same Price On Amazon .

https://www.amazon.com/Automotive-Smoke ... r=8-1&th=1
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[jreed]

Another place to check for leaks, cracks and splits is the intake tube running from the MAF to PCV/PTC inlet and to the turbo.

[abscate]

230F is about the highest temperature the coolant should get before the fan kicks on and runs it back down below 212 - it should go back to about 194F or so. I dont think it should go to 230 F at idle though - that should take some hard running to get there.

The dash temp gauge should be thought of as a stop light

Green COLD
YELLOW 3 o/c position - NORMAL
RED - anything above 3 o/c

I did map ticks on my T5 somewhere in this forum....found it!!

viewtopic.php?p=586749&hilit=coolant+te ... ts#p586749

[Grantkat]

Agree with Abscate and Sveedy - I had a horrible idle problem...bought a smoke machine (simple cheap one) found 2 leaks... replaced the lines and boom smooth idle. and no issues since then.

[JoebobNLacey]

Typed up a long explanation, attached photos, went to preview, and somehow lost the whole post.

Pointing to vacuum leaks is not the panacea for every ailment with these cars.

Here's the solution to my problem, courtesy of Dave in the U.K. who PM'd me: it was a wiring short inside the engine wiring loom. It would only short once heated up. More specifically, one of the 3 wires on IAC circuit would short when hot, but not when cold. I was able to verify the short by testing between IAC wires and the corresponding pins on the ECU socket. Trying to attach pics here of the wire damage.

All I can figure is the 10 gauge ground wire for the cooling fan, which wire runs through the loom, overheated in a 4" section damaging its insulation, exposing copper, and that then caused damage to adjacent wires, allowing some of them to short to ground. The section in the loom where this occured is close to the back end of the A/C compressor. I found 6 wires with exposed copper, several more with insulation worn almost all the way through. One of the IAC wires was one with exposed copper. I removed the ground wire entirely from the loom and ran a replacement 8 gauge wire independently, outside of the loom. The rest of the wires cleaned up well, copper strands seemed intact and insulation seemed relatively decent beyond the small openings shown in pics, so I individually taped up each of the spots and reassembled the loom. Haven't gotten beyond that point because I'm taking the time to address some other maintenance issues (lower radiator hose and tackling the engine-side heater hoses since I'm already this far into things). So folks will have to wait for a verdict on whether this solves my problem, but given the exposed copper it seems pretty clear that this was the cause.

[scot850]

That is a really unusual fault. In nearly 30 years of owning P80 cars I have never heard of an issue like this. So I am amazed you were able to find the wiring damage. Well done to you sir!

That almost looks like rodent tooth mark damage possibly.

Neil.