Tech Q&A Part #4

Bogging Issues

Hey there, I have a 1997 Chevy pickup with a 6.5L diesel, and I’m having some issues, such as a lack of power when pulling my fifth wheel trailer, resulting in the engine-service light coming on. Also, when climbing into mountain passes, she bogs down, and it seems like the engine is starving for air (without the trailer on). So, I took it to local diesel shop and was told that the trouble code indicated a lack of air or possibly an air leak. I can’t locate an air leak, so I’m at a loss. Can someone help me!

Thanks,

Dale Wickes
Via Email

Hey Dale,

Sounds like a turbo boost pressure problem, most likely a DTC-P0236, Turbo Wastegate Solenoid Fault.

The Diagnostic Trouble Code P0236 (1996+) is a somewhat common trouble code affecting the electronically injected 6.5L turbo diesels. Typical symptoms of a boost-related problem include an SES (Service Engine Soon) lamp illuminating when the engine is operating somewhere at or above 2,500 rpm and when accelerating. The SES lamp will usually go out once you slow down or reduce throttle. You may also experience a loss in power and see more black smoke at the tailpipe.

With the engine at idle, the vacuum should be holding the wastegate actuator control rod on the turbocharger closed, and it should take considerable finger pressure to move it open. If the wastegate lever is easily moved, you have a loss of vacuum or a problem with the vacuum control system – namely the Wastegate Solenoid.

With the engine off, you should be able to move the wastegate actuator control rod up and down fairly easily. A sticky wastegate (due to rusting) can be a problem if vehicles aren’t driven for long periods of time in a moist environment. For any boost-related problem, always begin your troubleshooting by measuring the vacuum at the wastegate actuator on the turbo with the engine idling. While most systems will produce approximately 20 inches of vacuum, GM specifies 15 inches of vacuum as the minimum.

With that said, they’re the two most reported failures that cause what you’re reporting: a bad Wastegate Solenoid or a leaking vacuum line. The most likely cause is the wastegate solenoid (mounted to a bracket above the driver’s side valve cover). The vehicle Powertrain Control Module (PCM) modulates the amount of vacuum seen at the wastegate actuator on the turbocharger by controlling this solenoid. The computer turns it on and off many times per second using a modulated square wave. The plastic vacuum lines snaking across the engine become brittle over time and are easily broken. You can bridge a break with neoprene vacuum hose. Easy fix. Good luck.

Diesel Motor Oil

I am a parts manager at a Chevrolet dealership. I have a question for you regarding CI-4 motor oil availability.  Here is a little background—I used to buy the 15W-40 CI-4 oil from GM—till a few years ago.  They discontinued that product and recommended we use the more expensive API rated CJ-4 to replace it.  My service manager and I decided to spec out some new CI-4 oil for our customers who still drive the 2007 classic and earlier diesels (non exhaust particulate filter equipped).  It has been suggested that the earlier CI-4 or CI-4+ oil was better suited for these “older” applications, and it is cheaper.  We had been using the Exxon XD-3 CI-4 oil until I recently learned that it is no longer available.

My questions: Is all CI-4 oil being discontinued and replaced by CJ-4? Do you know of any manufacturers that are continuing to produce CI-4 oil?  Should I spend my time finding a CI-4 oil only to have it be discontinued within a few months? I was told that the CI-4 or CI-4+ is better suited for the 2007 old style and older diesels by an aftermarket synthetic oil rep, do you agree with this statement? My goal here is to provide my customers with quality oil at the best price I can get it for.

Thanks for your time,

Bill Blant
Via Email

Good questions, Bill.

All players in the motor oil business have upgraded their diesel-rated motor oils to include the newer CJ-4 API rating. Unless you find some old stock, you’ll not likely locate a diesel motor oil that doesn’t include CJ-4. 

The Duramax can run from 5,000 to 7,500 miles between service intervals, which shouldn’t produce a huge impact on service costs, even with the slight increase in the cost of CJ-4.

Whether ad hype or not, the major motor oil companies advertise a number of benefits when using CJ-4 in any diesel engine when compared to the earlier CI-4 oil, whether it’s equipped with a diesel exhaust particulate filter or not. I recommend the newest oils.

3,000 RPM = More Performance?

Hiya Bill,

I have a question about performance, and I thought you might have some advice. First, I’ll tell you that I have a 2005 Dodge, which has an aFe Stage II cold air intake system, MBRP 4″ Cool Duals exhaust, FASS fuel system, and TST performance module. I’ve also installed a performance clutch and had the transmission checked out.

It runs fine now, but I want a little more. The one thing I have noticed is that it only pulls hard to about 2,500-2,600 RPM then flattens. If the engine would pull hard all the way to 3,000 or above, I believe performance could be better. What do you suggest?

Thanks,

Bart Gorecki
Omaha, Nebraska

Hi Bart,

At least four factors combine to shape the power and torque curves produced by these engines – boost pressure curve, diesel injection fuel-rate curve, bore and stroke geometry and the basic thermodynamics of diesel fuel combustion. Learning a little more about how the Dodge Cummins and light-truck diesels in general operate might help to explain what you’re seeing.

A turbocharger can be thought of as a torque adder. Generally, the engine’s torque curve closely matches the boost pressure curve across the engine RPM range. If you can alter the boost pressure curve to come in sooner or later, the engine’s torque curve will also come in sooner or later. Raise the overall boost pressure, and the engine’s torque curve will increase overall, provided you make changes to increase fuel delivery.

Most diesel fuel injection systems used by light trucks today provide their highest fuel-rate at somewhere around 2,000 RPM, which then tapers off at higher engine speeds. The maximum fuel rate is not uniform across the entire engine RPM range. This is done by the diesel fuel system manufacturers, in part, as an acknowledgement that diesel fuel needs time to burn efficiently in the combustion chamber. Time becomes shorter as engine speed increases.

The comparatively long stroke and long connecting rods of the Cummins work best at producing torque at a lower engine speed than a gas engine might. The V-8 diesels used by Ford and GM use a bore/stroke ratio closer to 1-to-1. The effect of this is that the V-8 geometry can result in a somewhat higher RPM torque and power peaks, and that the Cummins is more efficient when spinning a little slower. This isn’t a suggestion concerning which is better, but an acknowledgement of the design, which can be useful when defining powertrain management for best towing performance, track performance, or fuel economy. For illustration purposes, the Dodge 6.7L Cummins was originally rated at 350 hp at 3,000 RPM and 650 lb-ft torque at just 1,500 RPM. Gas engines have horsepower and torque peaks much closer together.

Dodge 5.9L Cummins – Bore 4.00 x Stroke 4.72”
Dodge 6.7L Cummins – Bore 4.21 x Stroke 4.88”
Ford 7.3L Power Stroke – Bore 4.09 x Stroke 4.17”
Ford 6.4L Power Stroke – Bore 3.74 x Stroke 4.13”
Ford 6.7L Power Stroke – Bore 3.90 x Stroke 4.25”
GM Duramax 6600 – Bore 4.05 x Stroke 3.89”
GM/AMG 6.5L – Bore 4.06 x Stroke 3.82”

Through the years, I’ve seen 1,800 RPM quoted time and time again as the best engine speed for maximum diesel fuel economy and which has been proven time and time again by industry tests. This isn’t a coincidence but rather a statement about the thermodynamics of compression ignition and light-truck diesel engines in general. Whether Dodge, Ford, or GM, all produce the best fuel economy when operated somewhere near 1,800 RPM. Incidentally, you’ll also find the same RPM relationship when working with any of the newest automotive diesels, like the Chevy Cruze or Volkswagen diesels.

Now, taken together, all of these factors combine to produce a Dodge Cummins diesel that produces the best combination of power and torque (performance) in the 2,600 to 2,800 RPM range—whether on the dyno or at the track. Dodge has widened the engine’s RPM operating range considerably through the years due to its adoption of electronic diesel fuel injection. The first-gen mechanically injected Dodge Cummins operated at a maximum governed engine speed of 2,700 RPM, while the current governed engine speed has increased to 3,500. Dodge did this to improve drive-ability. But, best performance remains in the 2,600 to 2,800 RPM window, even now with the new high-pressure common rail fuel injection.

So, to improve Dodge Cummins performance above 2,800 RPM, you’ll need a boost/fuel program that attempts to maintain the fuel-rate and boost pressure through 3,000 RPM or higher. This is where a variety of stock parts begin to struggle.

The factory turbocharger can’t efficiently provide that peak boost pressure much above 2,800 RPM. It’ll go higher, but efficiency begins to affect engine performance. The compressor efficiency struggles at higher airflows and pressures, which creates higher boosted air temperatures. Higher boosted air temperatures increase combustion temperatures and reduce air density. You lose one horsepower for every 10 degrees increase in intake air. The variable nozzle turbine, which was designed for typical drive-ability and to maintain low emissions, becomes increasingly restrictive to exhaust flow at higher engine speeds and exhaust flow rates. This can cause exhaust temperature to increase above the safety zone when making more power, and is made worse by an increasingly hot intake air. This is why aftermarket turbochargers and more efficient intercoolers are popular with owners who modify their trucks for better performance.

Monitoring fuel-rail pressures while making big power can tell you whether you need a second CP3 high-pressure fuel pump, a performance-oriented lift pump, larger fuel supply lines, or all of the above. Use a scan tool to look at the “commanded” and “actual” fuel rail pressures during a run down the track or while on the dyno. If less than about 10-kPa pressure differential can be maintained at maximum power, you should be fine.

As a way of providing a little more background for this mystical 2,600 to 2,800 RPM diesel phenomenon, some modified Duramax engines running highly specialized programming can spin to as high as 5,000 RPM. This can be advantageous to allow stretching out lower differential gearing in the ¼-mile or at Bonneville, but again, the best modified engine performance remains in the 2,600 2,800-RPM range – even for the Duramax. It pulls harder at the track or on the hill in that RPM range.

Generally, it’s not wise to stack more than one fuel injection timing device due to the possibility for unexpected consequences. Too much timing advance can be harmful to the engine. Talk to TST as well as the various performance vendors advertising here in Diesel World. They’ll be able to help you choose products that are stackable or provide products that meet, or come closer to, your expectations. Let us know what develops with your truck upgrades. Interesting subject. Thanks for writing. DW

If you like the Q&A series, here’s the previous post in the series: Tech Q&A Part #3