Hard Hot Starts, Lack of Throttle Response

[Blondie]

Im trying hard to keep your thinking hats on!

The car fires up beautifully after a few hours of sitting, but it gets harder and harder to keep the car running with every startup after that.

Ie. 1st try the car starts just fine...no missing, just smooth controlled running.

2nd try (5 min later) the car is having trouble keeping the idle above 800rpm

3rd try (5 min later) the car takes a few cranks to get going but eventually stays running...however its beginning to feel like a cylinder or two may not be firing...idle is at bone-rattling 700rpm

4th try (5 min later) I'm cranking steady for a good 10sec. while the car fires sporatically and usually doesn't come alive.

Also note that throttle response is usually limited to the first startup....if I get the car to run again after that, the engine wont respond to throttle position in any way, shape or form no matter how I mash it.

Last night I had the car running decent....was actually considering take her out for her (our) first EVER drive. Got this one last hurdle to tackle, guys!!

- Matt

[mjadair]

Have you checked for spark during this time? I have seen coils fail after they get hot. I would start here.

[Blondie]

Hmm.

Thanks for the reply...dont often hear of coils failing...but I won't rule that out at this point. I'll scope that out and report back.

[Blondie]

Coil resistances/voltage check out just fine..but the spark doesn't seem as 'hot' as it does during the first round.

I started the car and ran it for a solid minute or two. It ran great, idled fine but 0 response from the throttle.

I checked the coil resistances, they were still right on. In fact they barely changed. Voltage was within spec.

I then threw on a set of extra spark plugs and layed them out on the manifold. Maybe I'm paranoid but the spark seemed to get weaker as cranking progressed. (Yes, I'm aware that cranking drains the battery lol)

So where do I go from here?? :s

[Icey1174]

I just put a fuel pressure gauge and a new adjustable fuel pressure regulator on from Lindsey Racing. My stock FP was 38PSI. When I put the new FR in it was set at a low fuel pressure, around 18 PSI. It bearly started and died right away. I adjusted to about 30 PSI (you don't know the adjusted FP till you try to start it) and tried starting again. At 30 PSI I had the same symptoms you are stating. Once I was back to 38 PSI everything was good.

My sister also had this same type of problem with her Mustang. The garage she towed it to changed the fuel filter, put in a new fuel pump, and put in a new fuel pressure regulator. Problem gone. I don't know what the length of time the car ran for, or what the temp of the car to do with it, but it did effect her car the same way your car is acting.

After my experience of putting on the new adjustable regualtor and knowing first hand of the problems with my sister's car, I would be more prone to verify that everything in your fuel delivery system is in good working order with proper pressures. A lack of fuel pressure would give you not enough fuel to go to WOT. Can you check your fuel pressure with a gauge? If not I would think a local garage could check it for you rather easily. It took me less that 5 minutes to mount my gauge right to the end of my fuel rail. From the sounds of it you have pretty much ruled out a spark and coil issue.

Are all your injectors firing properly? I am not sure if heat and time would effect them.

I am just throwing out some suggestions here. Maybe someone can help that has heard of or had this exact problem and knows what the correct, quick fix is.

[Blondie]

So your thinking my fuel pressure could be falling the longer the car is run? Its a good theory..definately fits the symptoms

This obviously seems to be a heat related issue so...how would a fuel pump act when hot? And what would cause a pump to overheat?

[Icey1174]

Liike I said, I don't know for sure. She has a Mustang, not a Porsche. When it comes to this stuff a car is a car to me. A fuel pressure gauge would rule out a fuel delivery problem. Sounds easy enough, espically if you have a gauge! Then you could at least rule that out.

Are you the same one that had the video of the fuel injectors firing on here? I remeber an old post about it, but didn't have any real input. Gas, air, spark. One of them is not working, or mixing right. Rule out the easiest ones first.

[Blondie]

lol Yea, that injector video was mine.

I have a fuel pressure gauge (which is how I know that the pressure from the pump is good) so I'll throw that on there and see what the pressures like during the 2nd and 3rd attempts to start the car while hot

[Blondie]

Just ran with the FP gauge on the end of the rail. FP is peachy keen while running..

After about 20sec. the engine just died. I mean died like someone turned it off.

"Like someone turned the key" is the first phrase that popped into my mind and I remembered reading that in this thread:

Engine dies . . .


VERY similiar symptoms to whats described by this fellow. I'm wondering if the heating off the engine is pulling the reference sensors away from the engine and making it harder and harder to start/keep it running until the block has cooled/shrunk and pulled the sensors back into spec?

[mjadair]

When it died did you check the spark?

[Blondie]

Both the speed and reference sensors ARE working (beautiful idle). The first time the engine runs, it cuts out after 30 seconds or so. Nice, clean cut like someone turned the ignition off.

But the more the engine is started and ran after that, the sketchier it gets until it eventually bogs out and cant be kept running anymore.

[Blondie]

UPDATE:

After any start but the first one, the car will stay at a steady idle by keeping the pedal partially down (about 1/4 of the way). Any more throttle and the car stalls. Any less and it dies.

FP is definately good during every run.

[Icey1174]

All the fluilds good and full??? Water pump and oil pump pumping? You have spark, you have air, your have fuel, all the sensors are good... maybe it's something mechnical with the heat and expansion. Oil or water???

How long have you ben having this problem? Did you make any repairs or replace any parts before this started to happen?

[nynor]

vacuum leak?

[Blondie]

Icey:

This problem started yesterday. I got the car running like a champ on June 1st after a full year of restoring this 944. A previous lack of throttle response was solved by bending the tabs inside in the AFS downward so they once again made contact with the black strip.

Again I've lost all throttle response but this time it doesnt appear to be the AFS. The running problem started yesterday where the car will idle fine if its sat for a while, and get progressively worse every start after that.

Nynor:

I've sprayed/tightened/snugged up the majority of the vaccuum lines EXCEPT for the ones under the intake. Note that idle speed is 930 - 950rpm so I tend to lean away from a vacuum leak.

[Icey1174]

Sounds like it is temp related for sure. A bad or disconnected O2 sensor couldn't cause something this bad could it?

When you say "restored" it, do you mean engine work?

Almost sounds like it could an over heating engine. So many bad memories from when I was a broke kid! So many reasons why engines wouldn't start, why the would just die... Any possibility that it is not getting the proper oil or water circulation? Maybe low on oil or water?

Good fuel delivery, good spark, good sensors, good AFM... wondering about a heat issue and expanding parts that are physicaly mechanicaly killing the engine at higher temps. Back to one of the old cars that used to overheat.

[Blondie]

When I say "restore" I mean everything lol complete engine rebuild.

Hmmmmmmmmm very curious ideas there..

I very recently just flushed the coolant out of her!!

I'm wondering if I didn't do it right..in terms of letting the trapped air escape out of the bleeder screw on top of the head.

After it's first solid 40sec or so of running great, the block/head doesnt feel that warm at all. The same day I did the coolant I also filled her up with 20w50 oil.

[Icey1174]

Wow... the causes could be endless! Not that I am doubting your ability, but pretty much every part of the engine has been worked on since it last ran well. I am just throwing out some past experiences with the same symptoms. Maybe one of the 18,364,856,245,324 possibilities will help.

Is there any way to verify proper oil and water flow through the engine? I just did my cam housing gasket. It's possibe to put it on upside down. It fits both ways. But if you put it on upside down you block off oil passages in that part of the motor.

[Blondie]

UPDATE:

I sprayed a little WD40 into a few of the electrical connections (reference sensors, temp sensor, TPS, AFM..) abt 2 days ago. This morning I pulled the connector for the temp sensor and noticed a ton of WD40 sitting in there...

So I left all the connections sitting out to dry and when I gave the car another startup this afternoon I had my throttle response back!

However, yet again the car mysteriously just cuts out after abt 45 seconds...the temp sensor SEEMS to check out ok with the engine a little warm. I have no clue...but its definately the DME killing the engine...it doesnt even TRY to stay alive. Like I said, its just like someone turns the key to OFF.

[Dave101]

If the temp sensor is cheap, I would replace it. The rest of the engine, is new may as well have a new temp sensor as well and then if it doesn't work at least you can rule that out.