You won't see the thermal valve unless you have a reason to go looking for it. If you look at the back pressure transducer it has one line that goes to the EGR (firewall side). On the opposite side (radiator side) is a line that goes down between openings in the intake manifold. That plugs onto one port of the thermal valve. The other port on the thermal valve has a line that plugs onto a port in the throttle chamber. It's tucked under the intake manifold so you feel it more than see it. The whole purpose of the valve is to block vacuum to the EGR valve until the engine has warmed up. Simple. If you pull the vacuum line off of the back pressure transducer (radiator side) on a cold start and you feel vacuum, the valve is defective.

Speaking of the transducer they can go bad too. If the diaphragm gets a pin hole in it, the hot exhaust gas from the back pressure will cook it in pretty short order. Look for a bubbled scorched looking cap.

I have the same problem on my 1989 Amigo 2.6FI <img src="/forums/images/graemlins/cheers.gif" alt="" />
Changed out the temp sensor to the ECM....
Problem gone!!!

Changed out the temp sending unit for the ECM and no change. It was a PITA to get to also. Anyway, I also removed the big hose at the Throttle Body and used my thumb to let more or less air in through the Auxilary Air Valve while plugging the nipple that the hose would normally go on. When I let it pull full air, it would behave just as before (start, go to high idle, then die). When I cut the air down it had trouble starting at all.

I was talking with my dad and he thinks maybe I am describing the problem with too much detail. <img src="/forums/images/graemlins/confused.gif" alt="" /> I am now going to describe the symptom as brief and to the point as possible and maybe this will trigger an idea from someone. <img src="/forums/images/graemlins/smile.gif" alt="" />

When cold, I start the engine. It starts perfect and goes to high idle at around 1500 RPM. Five to seven second into this it dies. It does not chug or stumble before dieing. It dies flat out just like you turned the ignition off or cut the fuel supply off.

I will now bite my tongue <img src="/forums/images/graemlins/ignore.gif" alt="" /> and not add anything else. If you want to see how I keep it running until warmed up or what it drives like during warm up you can click the link at the beginning of this thread. Thanks for the continuing input!

NOW you want to confuse us with simplicity! If you have a problem both starting and running it's source is the likely the same problem. You've got to remember in starting mode the EGR is mechanically locked out (with the thermal valve) and the O2 is locked out by the
temp sending unit via the ECM so you eliminated (providing you've followed previous advice) the EGR, the temp sending unit, the O2 sensor. You're pretty sure you've eliminated all potential vacuum leaks
(although that plastic tubing can crack and not show it and there's nothing obvious like a big hole in the air intake. So maybe it's electrical.
Two things: 1)Check the control relay and fuse clip for the fuel pump.
Check the fuel pump wires. More than one of us (me included) has discovered bad connectors back at the fuel tank or corrosion in a connection point. Relays are cheap so switching one out to eliminate a faulty one is not a big expense. The relay is supposed to feed off the battery at start then switch over to the alternator power to run. If you're dying within seconds of start it could be a relay problem or the alternator. There are posts that point out why most shops don't pick up an alternator problem on these engines... they're special 2) If your MAF was replaced maybe pop the connector and give it a shot of electrical contact cleaner and blow out the venturi with air. While you're at it pull the connector on the alternator and give it a shot of the cleaner too. Check grounds. All the electrical depends on good grounds and there's a bunch of them; on the block, on the valve cover, on the intake manifold. It's something simple. It's a process of elimination.

And another thing: if the thermal air by-pass valve on top of your thermostat body is opening you are running too hot. That's supposed to be the Hail Mary rescue to cut down the engine temp by bringing in cooler air directly to the throttle chamber. That opens at 180. Normal engine temp should be 160. I drove around for a long time thinking the temp guage at the halfway point was great and wondered why my feet would bake while driving in the summer. Changed the stat the pointer went to 1/4, the engine ran better and my feet stopped cooking.

I agree. It seems the problem should really be something simple. I just can't find it yet! <img src="/forums/images/graemlins/scared.gif" alt="" /> It only has the running problem until warm and then it runs well. This seems like it should be a BIG clue but I just can't seem to decipher it!

I have considered this (and I will check it anyway) but it seems that the problem would not go away after a few minuets of warm-up.

I will see about picking one up and switch it out. Maybe another U-Pull-It run!

I did have it tested on the vehicle and they said it was fine (at an actual auto electric/alternator shop). I have heard that this could be an issue, but once again, why would it work fine once warm and not give some of the other symptoms? (idiot lights flickering/dim, etc.)

I will check this out and see what I can find. Electrical is not my strong point! <img src="/forums/images/graemlins/confused.gif" alt="" />

I agree completely! I am just trying to rule one thing out at a time. Keep the ideas coming and I will try and tinker when I can with what limited knowledge I have! <img src="/forums/images/graemlins/smile.gif" alt="" />

It does not appear to be opening. I installed a new radiator about six/eight months ago and it has run at about one third on the gauge every since. This is even when towing. New T-Stat also.

when my truck ran <img src="/forums/images/graemlins/shiner.gif" alt="" /> it did the same exact thing, was very challenging to keep it running from cold start enough to get up in temp until she kept herself afloat. after temps got up, purred like a kitten. I never figured it out before I tore it all apart for other stuff.

with the idle system of the 4Z's, it can be just about everything mentioned above. I also thought it seemed like some fuel-cut situation going on, as the way it would die was so drastic.

I'm almost leaning towards the thermal vacuum valve is stuck wide open, so after the intital high idle start, the EGR snuffs it out. but then again, the transducer should prevent that if there is no exhaust pressure.

anyway a faulty vsv for the fuel pressure regulator could come into play with this scenario?
<img src="/forums/images/graemlins/confused.gif" alt="" />

Seems like I should be able to figure this out but, what is a vsv?

Ran into a very similar problem a few years ago with my '91/4ZE1: It turned out to be a bad alternator (with poor connections). Replaced alt & cables, problem went away. Something about starting and running on the battery until up to speed, then cuts over to the alt... and dies.

I know you said it was checked, how about the cables/connections?

Good Luck.

I have not really checked/cleaned all the connections. Since it is not exhibiting the usual symptoms, (such as idiot lights on dim while running) and since it supposedly tested fine, I have been focusing elsewhere. I am starting the process of checking and cleaning every electrical contact I can see, especially the grounds like RT1 suggested. I am sure it will take a bit of time. I have a good lead on a used alternator out of a working 4ZE1 for a small amount of $. I am thinking I will just switch it out and see what happens. It can't hurt since I am throwing parts at it anyway! <img src="/forums/images/graemlins/angry.gif" alt="" /> Thanks for the input. Keep the suggestions coming!

vaccuum switching valve.