Okay, I know the code and the likely problem, but where is the source? After replacing the head gaskets, everything is perfect - except for this CEL that pops on 5-8 seconds after starting the engine. I have checked and rechecked the vacuum lines, thinking something was inadvertently left open. All is as it should be. I've not seen this code before.

There is another two digit code 51 for the ECT sensor, "voltage too high" but this CEL comes on right away with a cold start. Besides, I've had enough bad ECT sensors lately to know they do not elicit a CEL on the dash, so I'm pretty darn sure this code 51 is indeed for the Vacuum Circuit switch.

Any suggestions about where to look for the problem?

 

51 O,C Coolant temperature sensor out of range indicated -40°C / circuit is open

This code is not related with vac. switch. Inspect ECT connector, cleane it if it is dirty ore corroded, check its resistance with DAVOM (30-50 KOhm on cold engine, 1000- 2000 Ohm on hot). The next step check voltage on connected ECT - measure voltage - about 0.5-1 V for hot and 2-4 V for cold. You will easily understand what is bad - wiering/connector ore sensor. Less then 10 minutes to diagnose.

 

Thanks Pablo for the reply. I too, first though it was the ECT sensor. The existing one was only 3 weeks old but failed the boiling water test - remained open. NAPA replaced it under warranty and I though sure I had the problem licked. The new sensor made no difference, the same CEL coming on right after start up.

The only vacuum switch I know of that was disturbed during tear down was the cannister purge valve (solenoid) that sits under the upper manifold. It was working fine before teardown and wasn't dropped or damaged. I've tried reconnecting it and tapping it to see if it is somehow stuck open but nothing changes. I replaced this solenoid barely two years ago and don't think it would go bad just lying on the workbench.

C'mon gurus, put on your thinking caps and help me out!

(BTW, the gaskets were worth the effort. Leaks are all gone, it idles smoother and has noticeably better pickup than before.)

 

how does the engine start and run cold and hot?
I'm with Pablo, measure ECT voltage hot and cold....specs? will rule out wiring, sensor and connecter all at once or tell you have a bad apple somewhere

any chance ECT was bumped, head, plenum and intake manifold are fairly heavy....ECTs are fragile....seen one damaged internally just by end wrench being dropped on it...they are not careful with these parts in shipment and on back shelves....I've seen the UPS clown throwing the boxes in the back door of parts store
damaged wiring? heads and intake manifold have sharp edges

were the hg leaking on both heads, Aero?
corroded thru or burned thru blown out?

 

I ran the ECT test this morning and all checks out fine. Reference voltage of 4.65 v cold, drops steadily to around 2.97 v as operating temp. obtained. I'm still thinking cannister solenoid as it is the only vacuum switch involved in the gasket exchange. Also, the CEL light always comes on predictably 8 seconds after start up, cold or hot. Its obviously sensing something wrong but the mystery is where. (I just can't bear the thought of having to pull the upper manifold again to get to it. I thought I would ask you guys to see if I was overlooking something obvious beforehand.)

The engine starts fine, runs fine and if the CEL indicator weren't on, there would be no reason to suspect anything was amiss.

The hg was leaking on both sides at the middle outside edges. The pass side worse than driver. They looked to be both just plain worn out from the constant heating of the exhaust manifold. They were thin and really tired looking on the edges. No sign of being burned or corroded.

 

If you suspect the cannister purge solenoid, you can pull and check it. Use a vacuum pump on the lines, and apply 12v to the coil. One of its lines connects to a little fitting on the throttle body. See what happens if you pull that line off. The engine might run a little rough getting some unmetered air.

Removing the upper plenum isn't all that difficult. Sure, that's where you have to remove most of the electrical and vacuum connections, but it's a lot easier than the lower. And since you just did it, the lines have not yet welded themselves to their fittings, so they should pull off pretty easily.

 

aero,
you had me digging
that code 51 definition you have for hi/lo vac. sw. is from older Ford MCU systems....does not apply to EEC-IV Ford systems

but still doesn't explain the code 51 when ECT voltages are correct?

i think there is a short to chassis grd, insulation break, on the ECT sensor signal return lead GY/R
unplug from ECT and disconnect cable from ECU....DVOM check sig. return lead to chassis grd....ideal is infinity, should be greater than 1 meg ohm

further reading
http://www.wpk.p.lodz.pl/~pawelzap/schematy/ford%20code%20reader.pdf

 

96-4wdr,
Your digging was correct. The manual for the scanner I use has an MCU section. I've not been there before this and did not realize it was for older models. I thought at the time, it was a different set of scan parameters and the code it threw out (51) seemed appropriate for the problem. Thanks for the link. Now that I realize I've been on a wild goose chase, I'm going back to square one and rethink this entire CEL problem.

You're right about the non-explanation of the real code 51 when the ECT is obviously working. I even tried that vacant connector in the off chance I mislabeled the real one - no change and the blank connector is almost too short to reach the ECT.

 

AeroC,
the ECT op temp voltage should be lower than 1v at 195d F op temp and below 4v at 75d F
damaged wire=high wire resistance

http://fordfuelinjection.com/?p=28

 

When I remove the connector and measure the voltage, I get 4.8v with KOEO. Somewhere, I ran across a reference voltage of 5.0v (+/- 15%) and this seemed reasonable to me. This morning, I cleared the residual code, disconnected the ECT and it still throws the same CEL and code (51- "ECT voltage too high"). I haven't changed any wiring during this operation, just unplugged and reconnected as necessary to move the wiring harness to one side. Why this is suddenly throwing out codes is baffling. Keep the suggestions coming!

 

I'm not sure, but ECU may be bad too. The best way - to swap ECUs and try againe. But wiring may be damaged too. Disconnect ECU and ECT and measure resistance between ECT terminals and ground. The prblem may easily be hidden there.

 

Dave:

What year is your van? '91 as I recall, but I'm not sure.

For that year, do you have 3-digit or 2-digit codes? I was thinking that you might have misread the codes (unlikely but it has happened before).

 

Tung,

Away on vacation? Good to see you back.

Its a 91 with 2 digit codes. This is driving me nuts! Today I swapped in another new sensor just for curiosity sake - same thing. I would tend to agree with both 96_4wdr that the voltage should be reading around 3.5 at ambient and with Pablo that the EEC may be suspect. After all, its the ECC that is furnishing the reference voltage of 4.8v and then telling me its too high.

Another unsettling incident after messing with it today. After testing codes for nearly an hour this morning, I started it up again and noticed it was stumbling coming off idle while still in neutral. As I put it into forward the engine suddenly died and would not start or even begin to fire. I put the code reader back on and found #21 - Either the ECT is out range cold but comes into range when warm OR there is a problem with the reading from the CPS (crank position sensor). Several more efforts to start with same result. It would crank and crank with nary a single fire - exactly what a faulty CPS would do. I finally disconnected the negative terminal for 20 minutes hoping everything would re-boot. To my surprise and consternation, the engine did start up but the CEL is also brightly lit. I hate electrical problems.

Now I'm thinking perhaps the EEC is on the fritz. I had absolutely no electrical problems at all prior to tear down. Heck, there are only a few sensor connections, 3 main harness connectors and the 6 injectors to contend with, how in the world can the EEC go wonky by simply disconnecting the harness? And yes, I did disconnect the battery before beginning any work.

Suggestions or commiserations accepted.

 

aeroC,
is this the rig that has been giving ECT problems for past several months?

i would look for a short to chassis ground on the ECM sensor signal return line. one of these will cause the ECU to throw odd codes and it's control of the engine and tranny to be unreliable
pinched or damaged insulation is the usual cause
there are several spices and connections in this lead between ECT and ECU
high sensor lead wire resistance that changes will also affect ECU performance

sensor signal return can NOT be at battery chassis ground

this may be what fritzed your last 2 ECTs

i would bet that problem existed prior to hg change

one other possibility is a spark leak into the sensor wiring

 

96_4wdr,
Its the same (and only) rig. Good speculation on your part, but I'm almost certain the previous ECT problems have been due to a drop in coolant level exposig the ECT sensor tip to high temperatures and subsequent damage. Bear in mind, other than leaking hg, the rig was running superbly. Never an electrical issue and routine code scans always came back "11" - no codes stored. I certainly appreciate your persistance in lending a hand in figuring out this problem. At this time, since nothing was disturbed other than the connections to the wiring harness, I can't see how shorts or poor grounds would just suddenly appear....... Well, wait a minute. After saying that, I now just recalled that on the driver side of the engine block was a multiple ground connection point. I did jack up the engine as far as possible, trying to gain room to access the lower #6 cylinder head bolt. I wonder if a ground lead was broken during that process.

Now you've got me curious but its late and this will have to wait until tomorrow. I'll check this out and keep you posted.

Dave.