Car idle problems upon starting

[Chris-H]

Have been searching the web for understanding of car idle control at start up. I own a Q45 1990 car with 200K miles on it. Have maintained the car and like the car. It runs fine. Recently replaced all the injectors, gas filter and air filter. Before this replaced the FPCU. The MAF has also been replaced a couple years back.

The car has an annoying start problem that has been with it for several months. It used to start up just fine, rpm up to 1500 and settle. Then it got in the mode where it would start, but the rpm would run down to 0 unless you held your foot on the gas peddle. Manually holding the gas peddle down for 5-10 seconds to keep it at 2500 rpm, then letting go did the trick. Now the related symptom has changed where it will not start unless you pump and hold down the gas peddle. After cranking the engine for 3-5 seconds, it starts and runs fine and does not require you to hold the gas peddle down for the 3-4 seconds. The problem is worse if the car has been driven and then let sit for 15-minutes to 4 hours.

I know there is an automatic priming when the engine key is turned on, but I don't know how it works and what sensor triggers it. It better not be part of the FPCU as Infiniti dealer just changed that unit 6-months ago when the car stopped running and there was no fuel to the engine.

I suspect a sensor failure because the previous symptom before it changed was the car started perfectly from cold start and from warm start (if it hadn't been sitting for more than 30 minutes or so). The problem always happened after the car was warm and I parked it for several minutes 30 minutes plus and the engine temp had just dropped a bit... not cold.

Ideas on what this is? I don't know enough about engine control and startup control to start looking. I have the maintenance manual.

Chris

[elwesso]

hows your fuel pump??? Thats the likely cause. i hope the infiniti dealer didnt replace JUST the FPCU!

[Chris-H]

Hi--Thanks elwesso.

The dealer did only replace the FPCU. The FP was not replaced. I asked the mechanic to check the FP and he said it was ok. Also, the initial symptom was occurring prior to the FPCU being replaced. I understand that the FP could be the problem, but I would think that other symptoms would be present besides poor idle right at cold startup and never after that. Also, the car NEVER starves if you pump the gas and it is running ok. In other words, the problem always occurs only when you start the car cold or start the car hot but it has been sitting parked for 30-minutes to 4 hours. And it NEVER occurs after the car is running and you turn it off and restart it. The car responds well on inclines and has plenty of power and zip stepping on the gas on the freeway.

To me, the problem has conditional written on it. Meaning the ECU or FPCU or whatever is looking at a sensor and saying don't control the rpm the car should idle at 600 now rather than the engine has just been started boost the rpm to 1500 and drop it slowly. I don't know how the modern Infiniti engine gets primed and always starts without the need to press the gas peddle. After starting, it always revs up to 1500 rpm by itself and then dropps slowly to 600rpm. I'm pretty sure whatever controls this is what is broken.

Again, I am not an expert on car controls. I can and will check the fuel pressure upon turning the ignition on and not engaging the starter. I believe the FP should come up and if I listen maybe I can hear the faint noise of it priming... but I don't recall hearing much. I know the FP is suppose to be quiet and it is.

Anybody else have an idea that might be related to sensor or control input to the FPCU or ECU? Maybe a connector somewhere not making good connection with temperature?

ThanksChris

[Q45tech]

The ecu uses BATCH INJECTION [all injectors fire simultaneously] to crank the ecu starts out on the 90 degree CAS signal UNTIL the rpms reach say 500 then the sequential mode is selected [at the same time the ecu switches to the 1 degree resolution port of CAS].

Start out with detailed fuel pressure measurements pressure before turn on, presssure as you turn key on, pressure as you crank, pressure as you idle at 1500.

Measure accuracy of real coolant temp sensor 30F overnight the sensor should read it ~~~.

Check [wiggle MAF, both CAS outputs,

[maxnix]

Think you need to measure fuel pressure vs. rpm first.

Strange that dealer would replace FPCU and not pump as failed fuel pumps are what cause FPCU failures 99% of the time.

Oh, and please tellus about your timing chain guides. Hate for you to lose it all the next time you start your car.

[Chris-H]

Thanks Q45tech & manix;

I use the car regularily everyday. Just got back from taking my son to school. Same thing this morning, key in ignition, crank the engine and pump the peddle twice and it starts and runs to 1500 rpm and then settles back. From then on no problems.

Keep in mind this is a bit different than the symptom when it first started months ago --- start the car, runs to 1500 rpm and then runs down to stop unless I hold the peddle down to keep it idling. Also, I noticed that if I manually idled it at 600 rpm with peddle for a while and then let the peddle go sometimes the car would idle at 100 rpm or 150 rpm and do it very smoothly. Then giving it some gas after 5-10 seconds, the idle would settle back at 600 rpm and all was ok from then on.

On timing chain guide, the mechanic at Infiniti warned me a couple years ago that he could hear timing chain noise. Not sure if true or not as I have tried to listen for it and I cannot hear anything except a slight noise at startup due to lubrication settlement to the bottom of the engine. This slight metalic noise goes away within a fraction of a second after the engine starts. Infiniti here in Honolulu is reluctant to rebuild an engine and their recomendation was to drop a new engine block into the car... sorry it's not worth it for a 1990 and over $10K in change. I will buy a new car before I do that. So far so good. If I loose the timing chain, its over. New car.

Another way to put it, is I love my car but I'm not stupid. The car is aging and if I can keep it going until its 20, thats good enough to recoup some of the upkeep I put into it.

I also have been through the lifter problem that some have reported on the forum. I did have a clicking sound on one of the valves when I started the engine. I manged to get rid of it by using a cleaner in the oil and changing the oil several times. Again, the cost to open the engine block and get down into the holes to dig out the lifters and replace them was unbelievable. And there is no way to know which one so one must replace them all.

Thanks to all of you for your time and advice. Now give me a few days to let me go check it out.

AlohaChris

[maxnix]

Same thing this morning, key in ignition, crank the engine and pump the peddle twice and it starts and runs to 1500 rpm and then settles back. From then on no problems.

One should never pump the pedal (peddle, while a homonym, is a verb) on a fuel injected car. Somehthing or many soemthings is or are clearly amiss.

Maintenance is an ongoing commitment to obtain safe and reliable performance. Once one thing is compromised or ignored, the other wear items quickly snowball out of control. Comparedd to parting with large amounts of cash or car payments (did that onece, never again!), maintaining a great car is cheap. But there is always a line, and it's different for different people.

[goody90q45]

Then it got in the mode where it would start, but the rpm would run down to 0 unless you held your foot on the gas peddle. Manually holding the gas peddle down for 5-10 seconds to keep it at 2500 rpm, then letting go did the trick.

I had a very similar problem when I bought my 90Q with 222k miles. It would search up and down between 1500rpm to 0rpm, sometimes stalling, before settling in at 650rpm and running fine.

The car had been sitting for over a year and had radiator spray on the engine. All the electrical connections had some level of corrosion. Cleaned them up with a toothbrush and brake parts cleaner, coated the pins in dielectric grease, and the engine purred.

Take a look at these connections: MAF, CAS, TPS, EGR solenoid, and the #1 and #2 injectors. Tighten up the female pins for a good tight fit, especially the MAF connection. This is a simple DIY that won't take longer than an hour. Good luck.

[Q451990]

I agree on the electrical connections that Mike mentions... At 200K your car is well overdue for timing chain guides - I've personally seen destroyed engines as early as 77K, although 120K seems to be more typical. The shop would not need to rebuild the engine - just pull the front cover with the engine in the car and replace the guides. Easier said than done - and not what their shop manual tells them. Almost everything is written with the first step being "remove engine from car" - but real-world scenarios have brough about solutions that are much less costly.

If your fuel pump has never been replaced - I would highly recommend doing so as preventive maintence. They typically make it to about 80-100K before frying the FPCU.

Also - do you know when the intake, plenum, and idle control valve were last cleaned? You may be experiencing extreme carbon build up if it's been 80K+

Man... what I wouldn't do to be in HI now... we're having our first night of sub-freezing temperatures.

Good luck!

Heath