2005 CTS 3.6L with acceleration problems

[BadCad]

I have 122K+ on my CTS 3.6L. I have made a few change recently to better the performance, I thought...

This model requires removal of the upper intake manifold to change plugs or injectors. What a pain!

Anyway, I installed E3 spark plugs and replaced the factory mufflers with Thrush turbo units. Shortly after this I started experiencing sputtering during acceleration at shift points. It seems to be just before it shifts. I need to put my foot into it for the sputtering to occur because in slow city driving, it is almost unnoticeable. Today, the problem (sputtering) is getting worse. It seems to occur sooner (mid-RPM) and continue longer. Gas mileage definitely decreased.

Since the check engine light does not come on I have not put the scanner on it. I have an Actron CP9180 scanner.

A few items that should be noted:

1) At about 100K miles I replaced the computer with a used one (but thoroughly tested by independent performance shop).

2) Shortly after replacing the mufflers (about 105K), the check engine light did come on. The fail codes were P0420 and P0430. I assumed that was due to muffler change. I reset the codes and light has never reappeared.

3) I am very hard on the vehicle. It is pushed 5 days a week 50 miles a day (to and from work) at high speed highway miles 80+.

4) Poor acceleration at any speed compared to performance at 100K.

5) Vacuum and fuel pressure are fine.

6) The higher the engine temperature, the worst the hestitation.

I am just not sure of where to start. I don not want to just start replacing everything.

Thanks for your time and guidance in advance.

[BodybyFisher]

First thing to do, REMOVE THOSE SPARK PLUGS, and put in AC DELCO

When you do what you did, YOU do the validation, by using ACDELCO, GM/CADILLAC does the validation and they KNOW that the engine performs with them.

I once used BOSCH +4 plugs in an 86 Corvette and its would not idle. Start with the plugs, they are probably the wrong heat range for your engine, based on your statement "higher the engine temperature, the worst the hestitation". The hotter my Corvette engine was the worse the idle was

This design is extreme, are they kidding?, "developed by researchers at leading engineering universities". If your OEM plugs lasted so long why add an UNKNOWN into the equation? Head design, flame fronts, detonation, fuel charge cooling are technical subjects and you have changed a variable that could have a negative impact. Its not just about making a spark, the cylinder head and piston was designed for a specific flame front and ignition location. As soon as I removed the Bosch and changed back to AC Delco plugs in the Corvette, bam, the engine purred. GM engines perform best on AC DELCO plugs, its been like that since the 50s as far as I know. Over the years I have heard of 'fanatics' indexing theri plugs so that the electrode faces a specific location in the cylinder to improve efficiency, that is how sensitive certain engines are. By the way, the E3 and Bosch+4 look very similar.

http://www.e3sparkplugs.com/

One other thing to consider, did you use either anti-seize compound on the plug threads or dielectric grease on the boots?, both should not be used, and both can cause misfiring. Also consider the PCM, we have seen them deflubiate (my made up word) when they get hot, what were your symptoms when you replaced it?. But I feel you need to begin with replacing the plugs to eliminate that probable cause

Welcome aboard

[BodybyFisher]

This a part of an article written by a good friend of ours here at caddyinfo. He was an engineer on the Northstar Powertrain design team, and we miss him badly and wish him well. These two paragraphs will illustrate how complex this stuff gets and how if you change a parameter you are mucking up the works

All high output engines are prone to destructive tendencies as a result of over boost, misfueling, mis-tuning and inadequate cooling. The engine community pushes ever nearer to the limits of power output. As they often learn cylinder chamber combustion processes can quickly gravitate to engine failure. This article defines two types of engine failures, detonation and pre-ignition, that are as insidious in nature to users as they are hard to recognize and detect. This discussion is intended only as a primer about these combustion processes since whole books have been devoted to the subject.

First, let us review normal combustion. It is the burning of a fuel and air mixture charge in the combustion chamber. It should burn in a steady, even fashion across the chamber, originating at the spark plug and progressing across the chamber in a three dimensional fashion. Similar to a pebble in a glass smooth pond with the ripples spreading out, the flame front should progress in an orderly fashion. The burn moves all the way across the chamber and , quenches (cools) against the walls and the piston crown. The burn should be complete with no remaining fuel-air mixture. Note that the mixture does not "explode" but burns in an orderly fashion.

There is another factor that engineers look for to quantify combustion. It is called "location of peak pressure (LPP)." It is measured by an in-cylinder pressure transducer. Ideally, the LPP should occur at 14 degrees after top dead center. Depending on the chamber design and the burn rate, if one would initiate the spark at its optimum timing (20 degrees BTDC, for example) the burn would progress through the chamber and reach LPP, or peak pressure at 14 degrees after top dead center. LPP is a mechanical factor just as an engine is a mechanical device. The piston can only go up and down so fast. If you peak the pressure too soon or too late in the cycle, you won't have optimum work. Therefore, LPP is always 14 degrees ATDC for any engine

[BodybyFisher]

Here is more info from the guru, its not likely that the geeks from the university did this type of testing on your engine/E3 plug choice

Spark plugs should be carefully matched to the recommended heat range. Racers use cold spark plugs and relatively rich mixtures. Spark plug heat range is also affected by coolant temperatures. A marginal heat range plug can induce pre-ignition because of an overheated head (high coolant temperature or inadequate flow). Also, a loose plug can't reject sufficient heat through its seat. A marginal heat range plug running lean (suddenly?) can cause pre-ignition.Passenger car engine designers face a dilemma. Spark plugs must cold start at -40 degrees F. (which calls for hot plugs that resist fouling) yet be capable of extended WOT operation (which calls for cold plugs and maximum heat transfer to the cylinder head).

Here is how spark plug effectiveness or "pre-ignition" testing is done at WOT. Plug tip/gap temperature is measured with a blocking diode and a small battery supplying current through a milliamp meter to the spark plug terminal. The secondary voltage cannot come backwards up the wire because the large blocking diode prevents it.

As the spark plug tip heats up, it tends to ionize the gap and small levels of current will flow from the battery as indicated by the milliamp gauge. The engine is run under load and the gauges are closely watched. Through experience techni-cians learn what to expect from the gauges. Typically, very light activity, just a few milliamps of current, is observed across the spark plug gap. In instances where the spark plug tip/gap gets hot enough to act as an ignition source the mil-liamp current flow suddenly jumps off scale. When that hap-pens, instant power reduction is necessary to avoid major en-gine damage.

Back in the 80s, running engines that made half a horsepower per cubic inch, we could artificially and safely induce pre-ignition by using too hot of a plug and leaning out the mixture. We could determine how close we were by watching the gauges and had plenty of time (seconds) to power down, before any damage occurred.

[joeb]

ran ok with stock plugs and used pcm. does not run ok now with new plugs. sounds like pcm is not issue. would lean towards plugs.