Took a deep breath today and turned the ignition on to initiate the self test on the EST system despite getting a battery voltage reading with the ignition key OFF.
The digital multimeter fluctuated between 0 and 12.5 volts, but I couldn't distinguish the codes.
So I hooked up a very low wattage test lamp instead of the DMM and interestingly, it didn't light up with the key off.
But when I turned the key on ... voila!... it gave off the code 21 - indicating a fault with the coolant temperature sensor. I repeated the test several times and every time was the same.
So a DMM is not a good instrument to use for this test. A little test lamp, or possibly an analog multi-meter is the go.
Next step was to test the coolant temperature sensor by suspending it in a pot of water while slowly bring it to the boil and measuring the resistance.
Bloody thing is spot on specification!
So where to now?? The wiring looks alright. All the fuses are intact. I wish there was more info available on this stuff. I've ordered the factory manual but I have a feeling that's not going to tell me either.
The problem is a lumpy idle. It's had a lumpy idle for a while but last weekend it got lumpier. It also lacks power on hills... but my previous wagon was an EL, so this baby has always seemed to lack a bit of punch.
Prior to testing the EST, I did all the usual things for a lumpy idle - plugs, leads, filters, dist cap, rotor, checked vac hoses, PVC valve etc. The head was reconditioned 10,000k ago.
The wiring looks very good... can't see any problems there. I am installing new battery terminals and will clean up all connections. It's just had a new battery too.
On the coolant temp sensor side, there's been another development today. I got another thermometer and re-ran the test. On this thermometer, the sensor's resistance is a bit outside the spec range at each temperature on the high side.
That means it's telling the EST the engine isn't warmed up... when it is... so the EST might be trying to advance the timing by 10deg as it usually does when the engine temp is less than 70deg.
The error code was probably generated when the EST wasn't getting a "warmed up" reading from the temp sensor, yet it knew the engine had been running for say 30min... so it says "something's wrong".
I've ordered another sensor and will test it side by side with the current one tomorrow.
Vacuum leaks can play havoc with the idle of these engines (I'm assuming yours is EFI). Although you have checked hoses, try checking the oil filler cap seal, and the dipstick seal (I kid you not!).
Traced my problem to a leak in the rocker cover gasket - it had the usual oil leaks, so I thought I'd take a long shot that it leaked vacuum as well. If you read the factory manual, no gasket is to be used, just sealer. So I turfed the gasket, and used some ultra blue sealer. No more idle probs, and fixed the oil leaks too!
Thanks for your suggestions guys. Yep it is a carb version. As you both said... time to get serious with all those vac hoses. They all *look* in good condition and are tight on their fittings.
But I replaced the PCV one yesterday with the exact same brand of tubing and the difference in the suppleness is huge. So maybe one of them has gone a bit hard somewhere and that's causing a problem. They're relatively cheap and easy to replace... I probably should have done that first.
Anyway, the new coolant temp sensor hits the exact midpoint in the specified range of resistance at each temp. The old one is outside the range on the high side.
So I fitted the new one and without starting the engine, checked the EST codes... got the same 21 error code for coolant temp sensor... yet the battery had been disconnected for over 24 hours?
How the hell did it store that? Even with its own internal battery... can't believe it would be still working after 14 years.
So did a few short trips of about 10min, idle's still lumpy (maybe a tad less but I could be imagining it). Rechecked the EST, the code 21 is still there.
Immediate thought is wiring... but again, they look perfect... two wires wrapped in black tape which go into the loom. Nothing rubbing anywhere... looks fine.
So I'm going to wait until this factory manual arrives... it's the special one for utes and vans 88-93. Really hoping it's got some serious detail on this EST system.
xf_xr6... your suggestion to short or open the temp sensor circuit is tempting... but I'm not game. The Haynes and Gregory manuals go on about the care required with anything connected to the EST... better wait until I know a bit more about it.
So that's it for today. Time to forget about engines for a while and watch a bit of football.
Received the Ford workshop manual today... it's good... worth the extra $ over the others if you want to do ALL your own maintenance.
Explains a lot more about the EST system and how it works.... although there appears to be some contradictory info.
It says if the coolant sensor is detected as faulty, the EST assumes an operating temp of 85deg. My lumpy idle problem occurs when the engine is warm.
So my EST box must be assuming the engine is warm and therefore NOT advancing the timing by 10deg as it does when it's cold... so that shouldn't be causing the problem.
Then, in a troubleshooting chart under "Poor Idle when hot", it says to check the "coolant spark control system"... which I assume is the coolant sensor. But why would it cause a problem if the EST defaults to 85deg ?
Anyway, the good news is it tells you how to remove the EST box which I was a bit nervous about... and it's simple. It also provides the pin-outs for the wiring connector.
And it also says, when the EST box reports a faulty coolant temp sensor (code 21), this indicates "short or open circuit detected".
So 'TopGhia'... time to quadruple check the wiring!
With the EST box removed, I can do an end-to-end check of the wiring to the ECT sensor. That will tell me if there's any shorting or poor connections going on.
Got a couple of hours spare tomorrow... this is going to be interesting.
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