Toyota Repair: starting/idle problem, vacuum leak, vacuum modulator

Question
QUESTION: Dear Ted,

I have a 1991 Toyota Previa, which I love very much and want to keep as long as possible.  It now has 220K miles.  I bought it in December of 2004 with 158K miles on it.  Since the day I bought it, it has had problems starting.  It used to take 4-5 turns of the key to get it cranked up until my mechanic suggested disconnecting the cold-start injector.  Now it usually takes 2 turns.  Only when it is under 40F outside will it crank right up.  The warmer it is outside, the harder it is to start.

In addition, in about the last year, I've begun to get a rough idle, which varies between it hesitating and being really choppy like a diesel.  This also varies with how warm the van is.  The warmer it is, the worse my idle gets.  When it’s over 80F outside, the idle is scary bad.  Since we've had a cold winter, my starting and idle have been okay.  I know once it warms up, they'll both be problematic once again.

My starting and idle problem are related, I think.  I've done so much in 2009 to alleviate the problem including new plugs, new plug wires, new EGR valve, new EGR vacuum modulator, new BVSV, new cat converter, new distributor, lots of injector cleaner and top engine carbon cleaner all to no avail.  Yes, the van runs great with all the improvements but it still idles rough once it's warmed up and it still doesn't start as it should.

Two possible caused for my problems are 1) a vacuum leak or 2) a faulty sensor or gauge.  My check engine light has never come on so I'm assuming my O2 sensors are okay.  A vacuum leak is the lead culprit unless you may be able to think of something else.

Thanks,

Carl

ANSWER: Possibly a vacuum leak but it would be the same all the time and not temperature related, I think the cvold start injector should be replaced first, this should help the starting, I would also recommend replacing the eng temp sensor for the ECU, if you can't located it let me know,  it seems that it would be a logical choice as for as the termperature related idle problem goes.

---------- FOLLOW-UP ----------

QUESTION: Dear Ted,

Thank you for your response.  I have a follow-up question and comment if you don't mind.

First I'd like to ask you if the "eng temp sensor for the ECU" is the thermistor, which senses the coolant temperature and is connected between ECU terminals THW and E2?

Also, one issue I forgot to mention in my initial post is that I recently discovered that all the coolant had drained from my reservoir.  I filled it back up to the "high" level and, after about a week, it was below the "low" level (not completely drained this time).  There are no noticeable leaks in my radiator and I haven't seen any coolant on the ground under my van.  My fear is that my head gasket has been compromised to the point where small amounts of coolant enters one of my cylinders once the van has warmed up and the gasket has expanded, thus accounting for the rough idle.  This, of couse, doesn't have anything to do with my starting problem and the van runs just fine, it just idles poorly after warming up.  Could this be a likely scenario?

Thanks,

Carl  

Answer
Yes the coolant temp sensor connections that you mention are correct. It's very likdely that it is a headgasket problems as these engines had problems with headgaskets as well as cylinder heads cracking, you can have it tested for an internal combustion leak at most repair shops, it's a simple test using a checmical attached to the radiator opening in a bottle and drawing vapor in from the coolant, this will test for any hydrocarbons in the coolant.