4.0 SOHC Bad cold starts: A Theory

[Greg_DonWindsor]

Living in Eastern Canada, My truck is often subjected to very cold winters, with temperatures sometimes plummeting to -25 to -30Celsius (-22F) for weeks at a time. My 4.0L SOHC engine during these temperatures experiences very poor cold starts. Upon startup in the morning, the engine will chug along at low rpm, sometimes stalling and requiring second startup, almost as though it is hunting for it's correct idle rpm. Tapping the throttle and holding it at 1500-2000rpm for a few seconds will allow the engine to find it's initial warm up idle speed of about 1500rpm, before it warms up and eventually settles at about 650-750rpm, which is it's usual idle rpm during warm temperatures.

Some background information about my truck:
I purchased it in May of 2010 with only 77,000 original kilometers on the odometer.

It has been treated impeccably since I've owned it, with no expense spared for parts and/or labor if I ever encountered a problem that I wasn't able to fix myself.

The upper and lower intake manifold gaskets have both been replaced.

There are no lean codes, vacuum leaks, air, fuel, or spark delivery issues. Plugs, wires, and aftermarket coil all installed within the past 12 months.

The colder it gets, the worse the cold startups seem to be.

Leaving the block heater plugged in overnight seems to slightly help the issue.

Occasionally, even during warmer weather, I can hear a little bit of timing chain rattle upon first startup.

Now at 167,000kms, and I believe that the left and right hydraulic chain tensioners for both cylinder banks have never been replaced. (I did not know about this when the manifold gaskets were replaced, else I would have replaced the front [ie: driver's side] tensioner when I did the gaskets).

This is my theory:

The hydraulic chain tensioners for the timing assemblies initially rely upon spring pressure only to tension their respective chains until oil pressure has built up enough to act on the tensioner's piston with hydraulic pressure. As the tensioners age, the spring's pressure has been known to weaken, which causes the chains to rattle.

Oil of course becomes much thicker at cold temperatures, therefore taking longer to build up the necessary hydraulic pressure on the tensioner's piston.

My theory is that the spring pressure in the chain tensioners is inadequate, and because of the cold weather's effect on the time it takes to build up adequate oil pressure on the tensioner, that this is the reason for my poor cold starts. It would also explain why tapping the throttle helps to get everything started, as the temporary load on the engine puts the timing chains in motion thus removing some of the slack, and the corresponding cold start issue.

I would be interested to hear what some of you who have replaced the tensioner devices have to report about your startups in cold weather. And perhaps what [MENTION=111113]2000StreetRod[/MENTION] might have to say about this theory, as I know his knowledge of the 4.0SOHC is unmatched by most.

 

[2000StreetRod]

Full synthetic oil?

. . . This is my theory:

The hydraulic chain tensioners for the timing assemblies initially rely upon spring pressure only to tension their respective chains until oil pressure has built up enough to act on the tensioner's piston with hydraulic pressure..

I agree. Anytime the oil pressure in the tensioner cylinders is low the chains can rattle due to weak springs.

As the tensioners age, the spring's pressure has been known to weaken, which causes the chains to rattle.

Even if the oil pressure is normal the chains can still rattle if the cassette guide is broken. The tensioner piston travel is limited and can't extend
far enough to remove the excess slack in a chain with a broken guide.

Oil of course becomes much thicker at cold temperatures, therefore taking longer to build up the necessary hydraulic pressure on the tensioner's piston.

Are you using full synthetic oil? If not, you should be.

My theory is that the spring pressure in the chain tensioners is inadequate, and because of the cold weather's effect on the time it takes to build up adequate oil pressure on the tensioner, that this is the reason for my poor cold starts. It would also explain why tapping the throttle helps to get everything started, as the temporary load on the engine puts the timing chains in motion thus removing some of the slack, and the corresponding cold start issue.

I disagree. The jackshaft sprocket pulls the camshaft sprocket on the traction side as the engine rotates. The tensioner is on the slack side. The only time that a weak tensioner can affect camshaft timing is when the engine is decelerating and the traction side has slack.

I think you should consider other sources for your poor cold start idle.

If you're not using full synthetic oil the hydraulic lash adjusters may be slow to pressurize resulting is excess valve clearance at cold start.

A high flow, quality oil filter with a drain back check valve is a good investment.

Airflow, engine coolant temperature (ECT), inlet air temperature (IAT), time from engine start are some of the factors that the PCM uses to compute AFR. The 2000 has integrated MAF and IAT sensors. Your block heater increases the ECT but has no effect on the IAT. Have you tried cleaning your MAF and IAT sensors with an aerosol MAF sensor cleaner?

Is there a possibility that snow is getting blown into the air intake and getting sucked into the engine when it starts?

 

[Greg_DonWindsor]

I agree. Anytime the oil pressure in the tensioner cylinders is low the chains can rattle due to weak springs.



Even if the oil pressure is normal the chains can still rattle if the cassette guide is broken. The tensioner piston travel is limited and can't extend
far enough to remove the excess slack in a chain with a broken guide.



Are you using full synthetic oil? If not, you should be.



I disagree. The jackshaft sprocket pulls the camshaft sprocket on the traction side as the engine rotates. The tensioner is on the slack side. The only time that a weak tensioner can affect camshaft timing is when the engine is decelerating and the traction side has slack.

I think you should consider other sources for your poor cold start idle.

If you're not using full synthetic oil the hydraulic lash adjusters may be slow to pressurize resulting is excess valve clearance at cold start.

A high flow, quality oil filter with a drain back check valve is a good investment.

Airflow, engine coolant temperature (ECT), inlet air temperature (IAT), time from engine start are some of the factors that the PCM uses to compute AFR. The 2000 has integrated MAF and IAT sensors. Your block heater increases the ECT but has no effect on the IAT. Have you tried cleaning your MAF and IAT sensors with an aerosol MAF sensor cleaner?

Is there a possibility that snow is getting blown into the air intake and getting sucked into the engine when it starts?

I have been using Amsoil or equivalent full synthetic oil since I've owned the truck. I use Royal Purple or Mobil 1 filters. MAF sensor has been cleaned recently, and rarely gets dirty due to my use of an Amsoil oil-less air filter. I built a custom heat shroud for the air filter, thus it is virtually impossible that snow could come in contact with the filter media. A quick under hood check confirms that the filter is clean and free of debris or any other obstruction. IAT I occasionally measure with my tuner, and in the winter it is usually equal to ambient air temperature, or at most 8 degrees F higher, even when the truck is not in motion. IAC valve is also new within the last year.

Although I doubt any of my timing chain guides are completely broken, it is a slight possibility.

 

[2000StreetRod]

cold start rattle

I wasn't suggesting that your guides are broken just because you have cold start rattle. I had it for a few seconds after replacing all of my camshaft timing chain related components. That's why I installed a pre-oiler.

Since you have a modified intake and exhaust system the problem may be with your custom tune. It's more difficult to tune for decent idle throughout the engine operating temperature extremes than to tune for high performance. Your scanner allows you much better instrumentation to determine the source of your problem. Does the engine idle poorly for the brief period in open loop after start up? Or does it stumble when switching from open loop to closed loop? Do you have a wideband AFR meter to monitor the AFR?

Good choice of engine oil, air filter and oil filter!

 

[Greg_DonWindsor]

I wasn't suggesting that your guides are broken just because you have cold start rattle. I had it for a few seconds after replacing all of my camshaft timing chain related components. That's why I installed a pre-oiler.

Since you have a modified intake and exhaust system the problem may be with your custom tune. It's more difficult to tune for decent idle throughout the engine operating temperature extremes than to tune for high performance. Your scanner allows you much better instrumentation to determine the source of your problem. Does the engine idle poorly for the brief period in open loop after start up? Or does it stumble when switching from open loop to closed loop? Do you have a wideband AFR meter to monitor the AFR?

Good choice of engine oil, air filter and oil filter!

I do have an aftermarket MAF as well, and use the Superchips Cortex tuner. It's "Heavy Load Tow" tune for 91 octane has been installed until shortly after I bought the truck. I had never even considered that this could be the cause though that's definitely worth looking into. For some time I have considered the superiority of the SCT tuners and their ability to be custom tuned by professionals who are familiar with the myriad aftermarket modifications done to these engines and their performance requirements across a broader range of conditions.

I think my first move should be to replace the tensioners. If this has no effect, I might consider looking into a different tuner as well.

Really appreciate your insight.

 

[2000StreetRod]

monitor startup

I would not replace anything until you monitor what's going on. Can your Superchips Cortex tuner datalog PIDs (i.e. IAT, ECT, etc.) or just display them in real time?

You didn't answer my question about the wideband AFR meter.

 

[Greg_DonWindsor]

I do not have a wideband AFR. Hadn't thought of that either. The tuner is capable of logging data, though the program that installs on my PC has never worked properly, and Superchips was not able to help me resolve the problem. So essentially all I can use it's diagnostic functions for is monitoring certain engine parameters in real time.

 

[coppe]

I don't get snow or ice. But a year ago I had really rough start ups, and it turned put to be a vacuum leak and a faulty injector. You say you don't have a cel code, does your check engine bulb work? A friend swore he didn't had a code, and it turned out that the bulb was out. I had considerable rattle, about three seconds every cold engine start. But when the injector and vacuum leak got fixed, the rough starts ended.

 

[Greg_DonWindsor]

CEL bulb works. My tuner also scans for codes. No codes are present that would indicate a vacuum leak.

 

[2000StreetRod]

very limited datalogging

I looked at the Superchips Cortex manual. It appears to have very limited datalogging capabilities - speed vs time or distance vs time.

PIDs appear to be only accessible via real time display and there is no mention if more than one PID can be displayed simultaneously. It seems to have very limited capabilities when compared to my SCT X3 which I purchased for the same price ($379).

I doubt that your timing chain rattle is associated with your poor cold start idle. You might try monitoring your fuel trims from engine start. When the engine is started the PCM is in open loop using stored tables to adjust the AFR and ignores the O2 sensors. Once the O2 sensors have time to warm up the PCM switches to closed loop and uses the O2 sensors to adjust the AFR. The colder the IAT the longer it takes the PCM to switch from open to closed loop. There is an open loop flag PID and a long term and short term fuel trim PID for each bank. The fuel trims are bogus when the PCM is in open loop so ignore them then. If the engine idles poorly in open loop and well when switched to closed loop then I would suspect your tune is not matched to your engine configuration. The primary source of the problem is probably your 75 mm MAF sensor. The "Heavy Load Tow" tune probably assumes there is a stock 55 mm MAF sensor. The airflow of your engine at idle is the same for both MAF sensors but because of the larger diameter of the 75 mm MAF sensor it measures approximately 25% less air mass. There is a transfer function table in the PCM strategy that specifies air mass vs MAF sensor output voltage. I'm confident your AFR is lean when the PCM is in open loop. When the PCM switches to closed loop it adjusts the fuel trims using the O2 sensors to compensate for the lean condition.

 

[Greg_DonWindsor]

Here is a short video displaying some fuel trim pressure readings and a few other engine parameters while idling. The truck had already been driven for half an hour or so before parking and taking the video, so the engine was likely adequately warm by that point.

 

[Kiliona]

I would also suspect the tune as the first thing to check for an issue like this (as was previously stated)

Chances are the people who tuned it, when setting the tune, didn't have an adequate test in these conditions (being SO cold up there in caanada) and so just guessed on the tune at that cold of temperature, or even just set it at a flat tune passed say -10 or -20 (set a flat, or lightly increasing air fuel ratio without actually testing it to see if it was lean/rich at those conditions) since that's as cold as most people see.

It would be very hard to write a tune for a condition that you can't experimentally recreate (like it being so cold)... IDK about that tuner but a lot of places will let you data log then send them the log in order to adjust your tune, if this is an option I would try it if I were you.

 

[2000StreetRod]

no video

I couldn't view the video due to it being renamed or deleted according to the website.

 

[Greg_DonWindsor]

Sorry. Atempted to fix the link. Anyone else, trouble viewing?

 

[2000StreetRod]

Larger MAF sensor

Since I can't view the video could you post your fuel trims?

The main reason for increasing the MAF sensor size is to increase the maximum measured airflow. Some people estimate bhp by dividing the MAF measured airflow in grams/sec by .8.

1 lb/min = 7.56 grams/sec

As shown below the stock 55mm MAF sensor has a max reading (when voltage = 5.0) of 30.915 lbs/min or 233.7 g/s.

(233.7 g/s)/.8 = 292 bhp The stock 4.0L SOHC is rated at 205 bhp. Without forced induction it is unlikely the max capability of the stock MAF sensor will be reached. Your 75mm MAF sensor probably has a mass transfer function similar to the 80mm MAF sensor in the stock 1996 to 1998 Cobra shown above. For the 80mm MAF sensor the output voltage is 1.0 volt for about 1 lb/min of airflow. However, 1.0 volt in the 55mm MAF transfer function (used by the PCM) equates to only 0.726 lb/min of airflow. When in open loop the PCM calculates load using the lower airflow and adjusts the AFR and spark advance accordingly. The result is a lean AFR and excessive spark advance. Open loop occurs for a brief period after engine start (while the O2 sensors are heating up) and under heavy load (WOT).

Another argument for increasing the size of the MAF sensor is to reduce airflow resistance at heavy loads. This was the initial reason I replaced my 55mm MAF sensor with a Lightning 90mm MAF sensor. However, I also purchased a custom tune with an appropriate MAF transfer function.

My advice is to get a custom tune if you are set on using the 75 mm MAF sensor or reinstall the stock MAF sensor which is probably compatible with your Heavy Load Tow tune. The larger throttle body reduces airflow restriction possibly increasing max airflow at WOT but the stock MAF sensor will accurately measure the airflow.

 

[Greg_DonWindsor]

I'm not sure what the abbreviations stand for, but it displayed fuel trim pressure for both banks in STFT and LTDT.

STFT 1 osciillated from 0.8 to -0.8
LTFT 1 was 21.1%

STFT 2 oscillated from 1.6 to -1.6
LTFT 2 was 19.5%

I then displayed a few other parameters:
ECT 184 F
IAT 49 F (ambient temperature outside while measured was 15F)
Engine RPM 650 at idle
MAF flow rate 0.40 #/m (pounds per minute?)

I'm not sure what sh. Fuel trim and Ln. Fuel trim stands for, that is how the fuel trim parameters are labeled in my tuner.

 

[2000StreetRod]

Lean conditions

When in closed loop the PCM continuously cycles the short term fuel trims between slightly rich and slightly lean to control emissions. STFT 1 is the short term fuel trim for bank 1. Bank 1 is the side with number 1 cylinder (passenger side). STFT 2 is short term fuel trim for bank 2. LTFT 1 is long term fuel trim for bank 1. If the STFTs cycle above and below a significant negative or positive number the PCM adjusts the LTFT to bring the STFT mid-point back near zero.

Your STFTs are reasonable but your LTFTs are compensating for a significant lean condition due to your larger than stock MAF sensor.

 

[Greg_DonWindsor]

Thanks for all the info [MENTION=111113]2000StreetRod[/MENTION] since my current tuner doesn't have the ability to have custom files installed, I may look into upgrading to an SCT unit.

 

[2000StreetRod]

an SCT tuner won't solve the problem

Just purchasing an SCT tuner won't solve the problem. You'll also need a custom tune to replace your stock MAF transfer function (MTF) with one for the larger MAF sensor. Or you'll have to purchase the Advantage III software package in addition to the tuner in order to change the MTF yourself. The cheapest solution is to reinstall the stock MAF sensor.

 

[Greg_DonWindsor]

I intended to have it custom tuned as well. I've come this far with performance mods so far, no need to go the cheaper route. I've always read good things on this forum who have used the SCT X3 along with custom tuning.