High LTFT at Idle.

Hi, I have a 2003 1.6 zeetec ford focus petrol. The engine has a MAP sensor but no MAF. It has high emissions. It has high LTFT at idle. No codes have been set. The car drives and runs fine otherwise. The car has a history of short journeys.

O2 sensors are functioning as expected. I have looked for a vacume leak using brake cleaner and smoke in the engine. I could not find one.

The evap solenoid is closed at idle. If I pull the vacuum pipe off whilst the engine runs, causing a vacuum leak, the engine revs increase and the LTFT drops to zero.

I checked the exhaust for leaks and found the manifold leaks. I replaced this and re-tested the exhaust for leaks. The exhaust is now not leaking, but the LTFT remains at 25%.

I have cleaned the MAP sensor and this appears to work fine. The coolent temp sensor and throttle position sensor also appear to function correctly.

The PCV is in good condition and does not appear to leak.

What else can I check? Could this issue be caused by a leaky intake valve?

Thanks

I've seen valve timing cause fuel trim issues on these I believe. Are you sure this is a MAP engine? I don't think we have this option, on this year, here in the US. Pretty sure they are all MAF engines.
If it is a MAP engine only, a vacuum leak would normally cause a high idle speed. I am assuming that is not the case and idle is normal. This in itself would move us away from a vacuum leak.

Thanks for your response. Yes it is definitely a MAP only engine. The engine idles normally. A vacuum leak causing the engine to race makes sense. This is what I experienced when deliberately creating an air leak.

The LTFT is only high at idle and normal at higher revs. I had not thought of timing issues. Would timing issues cause symptoms across the rev range? The car did have a new timing belt before I owned it. I will check to see if this is out a tooth.

Thanks

I will be checking the timing belt marks on Monday. But in the mean time I have plugged the diagnostics in which gives the ignition angle for cylinder 1. This is 26DEG at idle. Im not sure if this is good or bad. Presumably this is controlled by the engine management based on sensor data.

I am waiting on the delivery of a pressure gauge so I can check the accuracy of the MAP sensor in more detail. The MAP reads 25KPA at idle. Do MAP sensors typically go out of calibration over time? What is their common failure profile? Is it possible the MAP is inaccurate at high vacuum? The MAP sensor in question also contains the intake air temp sensor. Part number 1s4a-9f479-ba3c13.

With a MAP sensor reading at key on engine tells you a lot it should be very close to atmospheric pressure at your location.

Great idea. I have carried out this test and it is 101KPA, so this suggests the MAP is good, however I will test this over a range of vacuum levels. If the MAP is out of calibration, the symptoms would suggest it's at the higher vacuum levels, if assuming it's faulty at all.

Thank you all for your responses.

Ant wrote: Great idea. I have carried out this test and it is 101KPA, so this suggests the MAP is good, however I will test this over a range of vacuum levels. If the MAP is out of calibration, the symptoms would suggest it's at the higher vacuum levels, if assuming it's faulty at all.

Thank you all for your responses.

The map values you report (101 kpa koeo) and 25kpa at idle are typical values and suggest that you're located at sea level....

What does "high LTFT" exactly mean, in numbers?

Map sensors do not suffer from "wear" and in my experience there is only a slim chance one will loose calibration on the low side only. With a simple vacuum gauge you'll be able to compare its readings to the map sensor.

If the Map reading was shifted then it would read lower than the real value, given your concern of positive fuel trim.(The 25 kpa at idle do not suggest this.) In this case the idle would be low and you don't state the idle rpm are low.

On a warm engine LTFT = +20. On a fully hot engine the LTFT shifts to +25. I guess this means that the problem deteriorates with temperature. The idle speed on a hot engine is approximately 800RPM. When the engine RPMs are increased, the LTFT drops. At 2600 RPM the LTFT is zero.

Tomorrow, I have access to an oscilloscope and I have made a exhaust pulse sensor. I will use this to see if I can see any problems with the valves. Although I have already checked the exhaust, I will recheck this, because I think the information above suggests a leak near the O2 sensor, thoughts?

One last point, I think the exhaust smells rich.

I will post back with what I find.

Regards

Update on my last;

I have retested since changing the manifold and I have different results. Also the weather is considerably cooler then when I last tested. Today it is 19 degrees C and I have a faster idle speed. It was previously 800RPM, today its over 900RPM. Im putting this down to denser air because its approximately 15 degrees C cooler then when I last tested.

The LTFT on a cold engine at idle starts at +1.6. As the engine warms to operating temperature the LTFT creeps up to 21.9 at idle. This seems to go up in line with the intake air temperature. On hotter days the LTFT creeps up further to +25 on a hot engine. Im guessing this is because the thinner air on a hot day translates to a slower idle speed on a MAP system??

I have taken a series of live data photos, starting with the engine off, then cold through to hot and then at higher RPMs on a hot engine. Im sure the answer is hiding in the data there somewhere! Please see attached. Apologies, I can't get them to load in order. The file names run from a to g.

Tabulated data attached. Starting with engine off through to engine hot at higher revs.