Misfire 3.8L engine - how to fix?

Tiny
JHAWKFRED
  • MEMBER
  • 2005 DODGE CARAVAN
  • 6 CYL
  • 2WD
  • AUTOMATIC
I have a 2005, 3.8L grand caravan with 75K mi which has again developed a misfire. Their are no electronic codes or indication with scope. The misfire is erratic but frequent when at 50 mph and slightly accelerating. At idle and under strong acceleration there is no miss. No carbon

This happened once before. The dealer finally pulled the head and found one valve with pitting. Regrinding and reassembling solved the problem for about 20K, just long enough that the extended warranty has run out. This feels like swapping parts without addressing the mechanism that lead to that valve pitting.

So assuming I have a similar issue with a valve or two, what is the root cause that should also be corrected/investigated? I do not want to go through this again. (E.G. What should I ask the mechanic to do to avoid it re occurring?) Any idea, whether the problem a malformed cam not properly timing a valve, weak springs, valve material of construction has too low a temperature range or strength, etc?)

The engine was tuned up 7K miles ago (8 mo - wires/plugs). There is an EGR error code this time, but not last time and the mechanic did not think that would be a root cause. Crank position sensor has not been changed. Have not checked yet for vacuum leaks, but wouldn't that show up more under hard acceleration? I don'f feel the misfire under strong acceleration at all.

Have not yet checked fuel pump pressure

If there is an upgrade for the problem, what brand or how to specify would be greatly appreciated.


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Wednesday, April 13th, 2011 AT 6:16 PM

5 Replies

Tiny
CARADIODOC
  • EXPERT
It sounds to me like the MAF sensor needs to be replaced becasue the signal is off sending a lean mixture to the engine. This is why is missing under load. I would also replace the ignition coil, wires and spark plugs with OEM parts.

Here is a guide to help you see what Im talking about.

https://www.2carpros.com/articles/how-to-replace-a-mass-air-flow-sensor-maf

and

https://www.2carpros.com/articles/how-to-tune-up-a-car-engine

and

https://www.2carpros.com/articles/engine-misfires-or-runs-rough

Here are the parts on Amazon.

https://www.amazon.com/gp/product/B007BLMWZW/ref=as_li_qf_sp_asin_il_tl?ie=UTF8&tag=2carprcom-20&camp=1789&creative=9325&linkCode=as2&creativeASIN=B007BLMWZW&linkId=b36058382b7fb8b3f451875e12fb324a

and

https://www.amazon.com/gp/product/B00KUL52NE/ref=as_li_qf_sp_asin_il_tl?ie=UTF8&tag=2carprcom-20&camp=1789&creative=9325&linkCode=as2&creativeASIN=B00KUL52NE&linkId=f6be88596252202094f74515ca0bfb9d

and

https://www.amazon.com/gp/product/B000CGK1ZQ/ref=as_li_qf_sp_asin_il_tl?ie=UTF8&tag=2carprcom-20&camp=1789&creative=9325&linkCode=as2&creativeASIN=B000CGK1ZQ&linkId=e3a4a59d6a4c308b3ab3eda9fcd75ee7

I have never heard of valve pitting being intermittent BTW?

Please let us know what happens.

Cheers, Mark
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Wednesday, April 13th, 2011 AT 7:46 PM
Tiny
CARADIODOC
  • EXPERT
Okay, ... I feel the urge to add a third dimension! :)

When you're accelerating, the fuel mixture in the cylinders is relatively rich. That means the fuel molecules are close together and it's real easy for the flame to jump from one to the next. When cruising, the mixture is much leaner. It's harder for the flame to jump to the next molecule. When it dies out before all the fuel is burned, you have a misfire. That is overcome by a really strong spark but that requires a higher voltage from the ignition coil. People smarter than me have run into problems from installing "higher quality" spark plugs. They might produce a hotter spark, but only if the ignition coil is capable of developing the higher necessary voltage. You might have a condition where the plug gap is a little too large or the plug design requires too much voltage. That's just another thing to keep in the back of your mind because that kind of spark plug trouble is generally only noticed when cruising at a steady highway speed.

Oh, and as far as the EGR valve sticking open, it is supposed to open at highway cruising speeds because that's where it's needed most and it's where you won't feel it. If it sticks open, say due to carbon chips, you'll really feel it at idle.
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Wednesday, April 13th, 2011 AT 7:58 PM
Tiny
JHAWKFRED
  • MEMBER
I will read these at home in about an hour and reply
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Wednesday, April 13th, 2011 AT 9:51 PM
Tiny
JHAWKFRED
  • MEMBER
They debated replacing the original pitted valve & seat, but chose to re-machine it. I may have misled you somewhat. The check engine light did come on, prior to taking it to the mechanic, with an ERG code (sorry do not have the code). The misfire itself has not caused a code or check engine light. The torque converter may better match the problem, as the miss has seemed worse in a very narrow speed/acceleration range. It looks like your suggestion of the OEM parts payed off, I replaced everything and the van has never ran better, thanks to you and this site :)

Fred
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Thursday, April 14th, 2011 AT 4:40 AM
Tiny
CARADIODOC
  • EXPERT
Glad you could get it fixed, that kind of problem can be tough. Please use 2CarPros anytime we are here to help

The original coil pack can deliver plenty of voltage to fire standard plugs with the correct gap. Sometimes people run into problems when they install a different type of plug in the belief they are upgrading to something better. In fact, the quality of ignition system components is so good compared to 20 years ago, there is no need to think about upgrading anything.

Platinum and split-fire spark plugs are the ones I hear about most often. When they are specified for a certain engine, it means the engineers tested their performance to be sure everything worked together. That can even include the angle the plug sits at in the combustion chamber and how the mixture swirls around near the plug when the piston comes up. Things like that can affect how a different style plug's spark reacts with the fuel. The plug can be a really high quality plug but it's design might not work in some engines.

A scanner is a hand-held computer that is connected to the vehicle to talk back and forth with all of the computers. Only the dealer's equipment is guaranteed to be able to access every computer and perform every function. There are a lot of aftermarket scanners that are used by the independent repair shops because one model will work on many brands of cars. One of the industry's biggest expenses is updating these scanners or buying new ones every year. That is partially the reason labor charges have to be so high. Aftermarket scanners never do as much as the dealer's scanners because the designers are always playing catchup, and they have to reverse-engineer so much stuff to figure out the software. Some aftermarket scanners are actually built by the same companies that build some manufacturer's equipment so they have an advantage.

The first Chrysler scanner, if you could call it that, was the DRB1, (diagnostic readout box version 1). All it did was read out the two-digit fault codes on some '83 model cars, nothing more. Mechanics rarely used it because Chrysler made reading those codes easier than on any other brand of car. All you had to do was cycle the ignition switch three times and count the flashes of the Check Engine light. GM was close in ease of reading codes, but you had to crawl under the dash to short two pins together, then you read the codes the same way.

The DRB2 could read codes, read sensor values, activate things that were controlled by the Engine Computer, (think radiator fan relay and AC compressor relay), and it could record live sensor data during a test drive. I have that scanner and the aftermarket version built by the same company. The Chrysler scanner only worked on Chrysler vehicles. The aftermarket version worked on Fords and GMs too but it didn't do as many functions or features. Independent repair shops liked the aftermarket one because they had to buy fewer pieces of equipment to be able to work on more brands of cars.

I also have the next generation DRB3 which showed up for the '96 models because of all of the new emissions-related stuff. Besides all the features of the older versions, it could be used to install new software into the vehicle's various computers. At first those downloads were received at the dealership over a satellite dish, into their stand-alone master computer, then transferred into the scanner which could be carried out to the parking lot and transferred into the car. The advantage was that if an emissions problem became known, the recall just involved updating the software vs. Replacing the Engine Computer. With different plug-in cards, the DRB3 will work on all Chrysler products back to the '83 models, and it can do most emissions-related tests on any brand of car sold in the U.S. Beginning with the '96 models. I have one of those too, but since I refuse to own a car that new, it gets used by my friends more than I use it myself. 1996 is when we started the government-mandated OBD2, (on-board diagnostics version 2) emissions systems. That system basically added the second oxygen sensor after the catalytic converter to monitor its efficiency, and it added leak detection to the fuel tank and supply system to monitor for escaping fuel vapors. That's why we see so many Check Engine lights now for a loose gas cap.

I get the next two scanners mixed up because they came along after I left the dealership. One is the Star Scan and the other is the Star Mobile. I get confused; it sounds like the Star Mobile is meant to take along in the car, but I think it means it is mobile and can be carried around the shop. One of them is meant to be left with the customer, I think, to use until the intermittent problem acts up and they can record the event for later examination. These scanners came about because the industry is trying to standardize much of the electronics and emissions systems on all cars, and they are using a new computer "architecture" rather than every manufacturer using their own systems. To the best of my knowledge, this first showed up in some Chryslers in 2004 on the Durango / Dakota. I don't know if it was in the minivans yet. No older scanners will work with this new system. Imagine if GMs spoke Spanish, Fords spoke French, Chryslers spoke German, and all aftermarket scanners spoke just enough of each of those to get buy on a basic level. Now we have agreed that everyone will speak Martian. That means that the tens of thousands of dollars worth of equipment that every independent repair shop bought will be obsolete and they will have to replace all of that equipment to work on the newer cars. This is why shops are charging over $100.00 per hour and can't afford to stay in business.

The latest generation isn't really a scanner and I don't think it replaces the last two. It is simply a laptop computer plugged into the car and a DSL line. Software is downloaded and updated from a web site. Chrysler is one of the most "customer-friendly" companies in that respect because they allow the independent shops to do that too. GM has always been the best company at figuring out how to separate money from their owners after the sale, and whatever they dream up, the other manufacturers usually copy a few years later. The problem is too many people have said "never again" when talking about buying another GM car because repairs are so insanely expensive and you must go back to the dealer. One of GM's latest tricks was to lock up all of their software and refuse to release it to the independent repair shops. Used computers from the salvage yard won't work because they have the vehicle ID number programmed in and will only work on that car. That means when one of their high-failure rate computers needs to be replaced, you must buy a new one from the dealer and have them program it to your car. Independent repair shops can buy the computer from the dealer and install it, but they have to tow the car to the dealer for the programming. Only three computers can be programmed by the independent shops because the government mandates they be allowed to do that because they affect emissions. There can be up to 44 additional computers on the car that the independent repair shops can't work on. (See why I will never buy another new car)?

Chrysler and Toyota allow any independent repair shop to download software for any of their computers except the Security System. That is to prevent vehicle theft issues. There is a small annual fee to access their web sites and a small charge per download, but at least we have access to it. Hyundai is the only company I'm aware of that allows access to everything by anyone for free. Isn't it funny that their sales are going up and GM's are going down?

Sorry for getting so long-winded. I hope you find the answers to your questions somewhere in there. To add one more point about scanners, you can buy "code readers" for somewhere in the neighborhood of $30.00 to $100.00 but they only read out the code number, then you have to look it up in a book. Since '96 those codes are standardized too among all manufacturers. Every one of those readers I've seen will only access Engine Computers, not the Anti-lock Brake, Air Bag, Body, Transmission, Memory Seat, Sliding Door computers or the Instrument Cluster which is also a computer module. I don't know if there is a good basic scanner for do-it-yourselfers that isn't too expensive. The DRB3 kit with way more stuff than you need for just one car model costs around $6200.00, but you are allowed to buy it. You can find a lot of other brands on eBay. Genysis is one brand hat a lot of people seem to like. The newest ones can be updated over the phone line. Anything made by Snapon is similar to GM products in that it costs a lot to maintain and update it.

I sell and repair car radios at the nation's second largest old car show and I sell the DRB3 and accessories there too. It isn't popular with do-it-yourselfers because of the cost, but the independent guys like them a lot because they can be used on so many other models and brands. It has a scope capability too. If you consider buying a scanner for yourself, be sure to ask at the dealership first if your van needs the DRB3 or the newer Star Scan. Only one of them will work and you want to be sure you're buying a scanner that "speaks the same language".
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Thursday, April 14th, 2011 AT 7:00 AM

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