rough idle/advance

beerhunter · Jun 7, 2018

Hi all

Having had an enforced and unwelcome break from playing with my truck, I'm now back on it.

It idles erratically. I have replaced the IAC, leaky fuel pressure regulator, fuel filter, and 02 sensor.

I disconnected the SPOUT, and set the timing to 10 degrees before. Interestingly (to me, at any rate), with the SPOUT disconnected, the timing mark held steady. However, when I re-connected the SPOUT, the surging recommenced, and the timing mark started jumping around.

I appreciate that the timing is supposed to advance when the computer is reconnected, but leaping around back and forth surely isn't?

Does this suggest that the TFI is therefore bad? Or does pulling the SPOUT kill the signal from all the other sensors, e.g. the TPS?

I think the temp sensor must be OK, as it idles approx. 1200 when cold, but drops to approx. 850 when warm (albeit with the annoying surge, which happens cold and warm).

Much obliged to you all.

p.s. I tested for a vacuum leak by disconnecting the lead from the IAC, and the engine instantly died - so presumably no air leaks

rusty ol ranger · Jun 7, 2018

Id say TFI or distributor.

RonD · Jun 7, 2018

Yes, TFI, but I would also say computer maybe............

Distributor issue should happen even with SPOUT removed

TFI system is a self contain unit, Module, distributor and coil
TFI can do centrifugal spark advance, for changes in RPM, but can't do Vacuum advance, changes in throttle position and fuel mix.

Computer has TPS and fuel mix data so can do Vacuum advance, and thats what the SPOUT wire is for, just like in the old days when you disconnected vacuum advance to set base spark timing.

So either the computer is sending the wrong info to TFI or TFI is misinterpreting that info.
Try unplugging the TPS and see if surging stops.

Also try getting the codes, there may be some History trouble codes that will point to something.

You can open up the computer and have a look at the circuit board, there are 3 blue capacitors that can leak after 20+ years, that causes odd issues, under $5 to replace them.

beerhunter · Jun 12, 2018

Many thanks for the replies. If it's not raining I'll have a play around tonight.

Related question - I had a look on RockAuto to see how much replacement computers are, in case that's what I need. They offer 2 for 2.9 manual trans (A-1 CARDONE 784412 and A-1 CARDONE 784795).

Their advice is to check the number on the one being replaced, to identify which is the correct replacement. However, the previous owner told me they replaced the computer, and given the way they've done other "repairs", I'm less than convinced they'll have fitted the right one.

Any idea on how I can check which I need?

adsm08 · Jun 12, 2018

Go with the numbers on the computer you have.

For a 1990 the likely difference is MAF vs Speed/Density, so if you have the wrong one it may well not even run.

beerhunter · Jun 23, 2018

Thanks to you good people, it's running again (with the clutch switch bypassed).

I disconnected the TPS, to see what effect it would have on the erratic idle - the truck cut out. Is that normal? I'm guessing it is.

Next is changing the TPS - can anyone tell me what size head are on the screws that hold it to the distributor? My 1/4 drive sockets are too fat to fit in the holes so I'll need to grind one down a bit, but would prefer to only butcher the one I need...

RonD · Jun 23, 2018

beerhunter said:
Thanks to you good people, it's running again (with the clutch switch bypassed).

I disconnected the TPS, to see what effect it would have on the erratic idle - the truck cut out. Is that normal? I'm guessing it is.

Next is changing the TPS - can anyone tell me what size head are on the screws that hold it to the distributor? My 1/4 drive sockets are too fat to fit in the holes so I'll need to grind one down a bit, but would prefer to only butcher the one I need...

TFI module is on the distributor, and engine can not run with it unplugged, it is the spark system

TPS = Throttle position sensor
This is located on upper intake manifold, toward the front, it has a 3 wire connector
It is on the opposite side from the throttle cable linkage

Engine should run without TPS plugged in

IAC(idle air control) Valve is on upper intake as well near throttle plate, it has 2 wires and engine should stall when it is unplugged, or RPMs would go very very low

beerhunter · Jun 24, 2018

Sorry, that was a typo. I meant I am trying to change the TFI on the distributor.

Interesting that the truck should have kept running with the throttle position sensor unplugged - I'll give it another try later when the distributor is back in.

2trux · Jun 26, 2018

The TFI uses a non-standard socket a 4.5mm I think. They sell a special tool to remove it that isn't too pricey.

beerhunter · Jun 26, 2018

Thanks - like most things, it's not sold in the UK, but I have ordered a set of sockets that include half-sizes, so will hopefully get this brute back together shortly...

adsm08 · Jun 26, 2018

This is what you really want:

https://www.amazon.co.uk/AMPRO-T71266-Ignition-Module-Wrench/dp/B00A8FO9OA/ref=sr_1_3?ie=UTF8&qid=1530035268&sr=8-3&keywords=Ford+ignition+module+tool

I have had mixed luck with regular socket sets. The problem is that not only is the bolt an odd size (actually 5.5mm IIRC) most sockets of the correct size have a wall too thick to fit the hole.

beerhunter · Jun 26, 2018

Thanks for that. If this new set doesn't work out, then I'll order one of those.

I wanna get stuck in and get this thing running right!

rough idle/advance

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