Hot-Start Issue: Rich vs. Lean...

[rossiz]

i've read a ton of threads on the hot-start issue and the general consensus seems to be it's a lean condition due to heat soak, vaporized fuel, hot injectors, low fuel pressure, etc. this all makes sense, but my experience is of an overly rich condition and it even fouled all my plugs.

the background:

'78 280z, 3rd owner, PO had it for 30 yrs as garaged DD.

when i bought it i thought it ran pretty well (first z car, so i had no idea what it could really do).

found leaky heater hose in cabin, flushed system and found much rust. replaced all coolant hoses, big and small throughout including heater hoses.

then turned to electrical: moved engine bay relays into cabin and replaced fusible links w/maxifuse, also mounted in cabin, new wire loom for all primary power including battery leads to starter.

next fuel system: all new fuel lines in engine bay, new filter between tank & pump, new pressure regulator, all new injector plugs/connectors, all new vacuum hoses in engine bay (dried/cracked), new pcv (gunked/stuck) and hose (burnt/rotted), new AAR (frozen), new AFM & boots, removed/blanked off EGR system (frozen/broken), new K&N cold air intake.

lastly, ignition: new plugs, wires, rotor, cap, set spark to 33 degrees BTDC max advance at 3,000 rpm, vacuum advance works perfectly w/no leaks.

after all this the car was transformed - i mean WOW!! and i spent a sunny saturday afternoon hooning around my neighborhood like a teenager thoroughly excited w/my new found performance. this z really moves!

ran a few errands and noticed that it started harder and harder after each stop. last errand was to pick up a can of seafoam. came out of the store 15 min. later, poured it in on top of a full tank of gass, and the car wouldn't start (?!?!) it would start briefly and die right away, then did start and spluttered horribly for a few seconds until i coaxed it up to above 2,500 rpm and then seemed to stabilize.

next day i took my daughter to an appointment (got it good and hot on the way there) and while waiting i pulled out my ipad (yes, i'm a dork) and researched the hot-start issue. i immediately ordered all new sensors for the thermostat housing, new cold start injector and new plugs/connectors for all of the above.

20 min later i went out to try and start the car and it would NOT fire up. at best, little 2 second runs then died, all the while exhibiting flooding symptoms (stinky exhaust, spluttering, lingering smoke/gas smell wafting out the tailpipe after it died). another 40 min later the appointment was done and when my daughter came out the car was still not starting. at one point (after MUCH cranking and cussing) it fired up and idled (horribly) for a minute or two while producing copious quantities of noxious smog - pedestrians were crossing the street to get away from it - then it died. i had no tools with me (a mistake i will never make again) and had to call for a tow (2 hr. wait). we walked to dinner and came back 1 1/2 hrs later and since it was a chilly night i gave it another try - it started but again idled horribly, belched smoke and wouldn't splutter past 1,500 rpm for anything. got towed home in shame.

pulled plugs and they were horribly fouled - a couple of them completely bridged with carbon. they had less than 20 miles on them and were now toast. new parts came, and as suspected all the sensors were covered w/rust & sludge on the wet side and corroded on the outside. the cold start injector was black and sticky - the PO had overfilled with oil, and i fear it was foaming and getting past the stuck pcv to gunk stuff up. replaced all parts, new connectors, cleaned the corroded bullet connectors too, and dropped in a new thermostat for good measure. new plugs and she fired right up (cold, though) and ran smoothly. i checked the AFM and fingering it showed that it actually wanted a little more fuel, so i moved the wheel 4 teeth richer. car runs great now, but still starts hard when hot and i'm terrified of fouling my plugs again.

any sage explanations for how a lean condition would foul plugs?

sorry for the loooong thread, but wanted to get all the info out at once.

[Zed Head]

Your problem sounds more like leaking injectors. They do that when they get old. The typical hot start scenario is not a starting issue but a rough-running problem after it starts. The engine starts right up but runs really rough. Fouled plugs and no-starts are typical of too much fuel getting in to the cylinders. You might just need new injectors.

Fingering the AFM at idle tells you what the engine wants at idle. There's an adjustment screw on the AFM for idle air adjustments. Adjusting the spring changes fuel enrichment over the whole air flow range.

[FastWoman]

I'm surprised you had enough carbon to bridge the electrodes! Was there this much carbon on all the plugs, or just some? With the hot restart problem, the theory (or at least mine) is that some injectors are spraying fuel, while others are blowing fuel vapor. This could happen with or without an underlying rich or lean condition.

I suspect your coolant temp sensor connection became faulty, causing an extreme rich condition, resulting in the extreme fouling. Whenever I have a hot restart problem, the car fires up without difficulty, but it seems to run on 2 cylinders until I can get past the "theoretical" fuel vapor, which is usually when I get to the road and punch the accelerator.

An extreme lean condition can result in fouling if there's incomplete combustion. I'd say if your engine is running lean enough to backfire, it's lean enough to foul the plugs. It's uncommon, but this is exactly what happened in my own Z. When I richened the mixture, the carbon fouling problem was gone.

[superlen]

Three possible scenarios I see.

1) Zed's theory on leaky injectors

2) Sarah's intermittent CLT sensor connection

3) Intermittent internal ECU issue

Cold start might have been in the running, but you said you had replaced it. I suppose it could still be an electrical issue firing it, but the time/temp elements of this problem don't support that. Let's ignore the CSV for now.

Of the three, I think the CLT sensor wiring is where I would look first. The problem could smells of be leaky injectors as well, but that's more of pain to test. I know you replaced the CLT sensor, & you said you replaced connectors, but did you just change the injector connectors or did you do the CLT sensor as well? That's a critical one.

Here's an easy test to learn something. It will require you get possibly stranded somewhere so drive around like a crazy teenager and get it good and hot then pull back into your driveway.

Now, try and start it and hope it exhibits the problem. Assuming it does, now pull the CLT sensor connector off (Make sure you distinguish between it and the temp sending unit, they look almost identical - I'm sure you figured this out already). Now try and start it. If it was any other problem other than the CLT sensor then your really hard to start car will now turn into an IMPOSSIBLE fully soaked plugs floating in fuel car. This is because you have leaking injectors/bad ECU/.etc PLUS super rich from the CLT sensor being disconnected. If however, you see no change and it's still hard to start but not impossible or at least much worse, that's a good indication that the connection to the ECU from the CLT sensor is suspect.

Len

[rossiz]

thanks for the replies - dumb question: CLT stands for what??

[siteunseen]

thanks for the replies - dumb question: CLT stands for what??

I think he means coolant temp sensor. It has two bullet connectors about 8" from the thermostat housing. Here's some pictures of the connections on top of the motor but he could mean where the wiring harness connects to the ECU.

http://atlanticz.ca/zclub/techtips/electricalconnections/index.html, 4th picture down.

[siteunseen]

So the Cold Start Valve does not squirt fuel anytime the motor is hot? Wouldn't that be beneficial on starting regardless of the temp? My Exmark mower has to be choked EVERYTIME I crank it, no matter if it's scorching hot. Not full choke but about half, always. I thought the CSV shot a little each time you turned the key.

[Zed Head]

You might have had two problems going on - the original plug-fouling and "copious amounts of smog", and now the more normal hot-start issue. The hot start issue doesn't usually produce the haze. It's one of the first things I noticed about the situation since I've had many old cars that used to flood occasionally. With hot-start the car runs like it's flooded or flooding but there's no copious gassy haze.

With all of the work you did you should just reset and see what the current state of affairs is.

The CSV is run by the Thermotime switch. The "thermo" part describes the bimetal strip that opens the contact when it's hot. There's also a heater wrapped around the bimetal strip to make it open after a certain amount of "time" with current running through it. So the engine doesn't flood when trying to get it started. It only gets current when the starter is turning. Kind of ingenious.

[siteunseen]

So if it gets current only with the starter, it's squirting some fuel when the starter is engaged? I'd like to think it gets a little shot on start regardless of the temp, it has to help.

[Zed Head]

Yes it squirts when the starter circuit is hot. The injectors also squirt a little extra, supposedly. It's shown at page 10 of the Engine Fuel chapter in the FSM. 3) "Start" enrichment.

[Captain Obvious]

I'd like to think it gets a little shot on start regardless of the temp, it has to help.

Not regardless of the temp... Only squirts while cranking below 72F max. (FSM says the thermotime cuts out somewhere between 57-72F). 77 FSM page EF-14.

[superlen]

Site,

Sorry for the CLT acronym. As others pointed out, it is the coolant temp sensor. I've been immersed in engine management systems design and algorithms for a few months now and assume that everyone thinks in those terms. I posted a quick list of the common terms for EFI sensors/parts below in case anyone is interested.

Captain/Zed described the CSV (cold start valve) in good detail. The important thing to remember is that it is 100% NOT controlled by the ECU. It is it's own little independent circuit & associated plumbing. Also important to realize is that the fuel pump relay is also NOT controlled by the stock ECU. It, as well, is just controlled by relay logic using the switch in the AFM and the start position of the ignition.

In short with the stock ECU no priming pulse and/or additional fuel shots are possible/configurable.

Len

-------------------------------------------------------

ECU/ECM - Engine Control Unit/Module

EMS - Eng. Management system

CLT - Coolant Temp sometimes CHT for Cylinder Head Temp like on an air cooled VW

IAT - Intake Air Temp

TPS - Throttle Position Sensor

MAP - Manifold Absolute Pressure (can be senosr or sometimes used a generic for speed density fueling algorhtym)

MAF - Mass Air Flow sensor

AFM - Air Flow Meter

EGT - Exhaust Gas Temp

EGO - Your O2 sensor

UEGO - Newer style wideband O2 sensor

BARO - Barometric Pressure