E30 intermittent start/dying iac problems?

[E30 manic]

So my 88 325is has 309xxx miles manual mostly stock is starting to have a problem starting sometimes and will randomly die, and restart most of the time immediately after but today it did not.

I cleaned the iac, have 2 spares in the car as well, have brand new ie plug wires and new plugs, I’ve adjusted the tps correctly, and have a know good cps.

So today in second gear at 15 mph the rpms dropped and it stalled. I couldn’t get it to restart and the battery got to low to turn it over. A coworker came with cables and we got it started eventually and I got to work.

The iac was not running at all, until I plugged in a spare tps and it started the iac. But the car still wouldn’t start and after 2 tries and iac wouldn’t kick on anymore. I plugged in my spare iac and played with the tps but they wouldn’t start running. So I cranked it over about 10 more times and had no fuel smell at all so I jumpered the fuel pump relay and it started right up. I left the spare tps plugged in and opened the throttle plate a little to keep the idle high in hopes it wouldn’t die again, and I made it to work.

What would stop the iac from running, I’m thinking that my tps is flooded with oil and wasn’t giving a good connection but the spare should run it without any problems and even then it didn’t start when I got it to run the first time.

Any advice would be greatly appreciated, thank you

[nando]

With the ignition on, the IAC will hum constantly (at around 90hz) - even if the engine is off or you're not idling.

Check that first. Could be wiring, but most likely the IAC is just dead. a broken TPS won't stop your car from starting.

[E30 manic]

I understand the iac should be humming with the key on but it would not hum until I plugged in my spare tps with the ignition on. I plugged in my 2 spares as well with the ignition on and didn’t get a hum until the spare tps was plugged in.

Before i wouldn’t get a hum from the iac if my tps wasn’t reading that it was closed, and adjusting or opening and letting the throttle plate snap close would kick it on. I didn’t have a mitlimeter with me this morning so I couldn’t test if my tps was reading closed or not or test voltage at my iac connection.

But let’s say that the tps isn’t telling the ecu that it’s closed so the iac doesn’t get the signal to start. Would that keep the car from starting?

thank you for the response

[E30 manic]

So I just got done driving the car around the block, it died once at a stop light and took a couple tries before it started. I parked it and let it idle for a bit and it died. Iac was still humming, it started right back up and died after 4-5 seconds. Again iac was still humming and it started up again and ran for 2-3 mintutes, died again with the iac still humming. Started right away again and stayed running for about 10 minutes before I turned it off.

Is this starting to sound like fuel pump maybe??
ive owner the car for about 10 years now and haven’t changed the pump out. But the car doesn’t usually bog down it just cuts out but when it died the first time today the rpms dropped out but the engine was still running.

[nando]

I don't think it's your IAC then. If it completely dies, it's silent.

Do you still have the original 3 engine relays? Main, FP, and O2 heater? I'd replace those as a matter of basic maintenance - a failing main relay can act balky (and does NOT prevent the starter from cranking), same with the fuel pump. And at 25 years old, they're likely toast (or nearly so).

[digger]

its not the IAC, the IAC would only cause no start perhaps if its really cold and fully shut but you'd be able to use throttle.

i had this exact symptom it was the 12V feed to coil dropping out momentarily with ignition on, if you get the situation where it cranks but wont start then see if there is spark. if there is see if theres fuel (injectors pulsing)

[E30 manic]

Yes I still have the 3 relays I keep some spares in the trunk but theyre used from the junkyard. Ill get replacement for those this weekend.

The thing about the iac is when it first died it wouldn’t start humming again until I plugged in the spare tps. I left the key on when doing this so I would feel it start humming right away.

I don’t think it’s the iac itself but something that turns it on and off or maybe the wiring.

How did you fix that/ how did you find that?

after work I tested continuity on the tps closed and open it tested good. I tested voltage at the iac plug with the key on and it tested good. I ran it for a bit and it didn’t die. I drove home with no problems at all maybe 6-7 miles, in stop and go traffic.