1989 Dodge Van Carburetor backfire

Hi onefatherslove. The place to start is by reading the diagnostic fault code(s). There is at least one set in memory since the Check Engine light came on. To read the diagnostic fault codes, cycle the ignition switch from "off" to "run" to "off" to "run" to "off" to "run" three times within five seconds, then leave it in the "run" position. If the starter ever engages, even for a second, turn the ignition switch off, wait a few seconds, then start the procedure over. After a few seconds the Check Engine light will display the two-digit codes as a series of flashes and pauses.

Count the flashes for the first digit. There will be a short pause, then the second digit will be flashed the same way. After a longer pause, the next code(s) will be flashed. The first code could be code 12. Disregard that one; it just means the ignition switch was turned off. The last code to be displayed will be 55. That means "end of message".

That will give us a starting point.

caradiodoc

Thank you for your reply. Here are the codes.
First is 1flash, short pause then 2, ( 12?)
Second is 1 flash, short pause then 5, ( 16?)
Last is 5 quick, short pause then 5 quick, ( 55?)
I hope this helps let me know if there is anything else I can do.
onefatherslove

That middle one would be code # 15. I don't have an '89 book for a full-size van, but the codes should be the same as for other vehicles. They list it as a missing speed sensor signal. If you have cruise control, and it works, the speed sensor has to be working. Regardless, that won't affect backfiring.

I can think of three things to look at. Your dscription of the symptoms is typical of a partially plugged catalytic converter. Excessive back pressure in the exhaust system causes hot exhaust gases to be forced backwards into the intake system. There is a tool for measuring pressure ahead of the catalytic converter, but it requires drilling a 1/8" hole in the exhaust pipe. If your converter has a 1" diameter pipe leading up to the top of the engine, you might be able to disconnect it to see if engine performance improves. If a lot of exhaust flows from that pipe, suspect a restriction in the converter.

Low fuel pressure will cause a lean condition which affects power and can lead to misfires. Chrysler has always had very little trouble with fuel pressure regulators but it's always a possibility. Throttle body systems run at a relatively low pressure, typically around 15 psi. If the system uses a vacuum hose to the regulator to vary system pressure, disconnect the hose at the regulator and plug it. Fuel pressure will rise and you might see black smoke from the tail pipe from running too rich, but if the misfiring clears up, you'll have a clue. For regulators that don't use a vacuum hose, you can use a hose pinch-off pliers to pinch the fuel return hose to the tank. That will also cause fuel pressure to increase.

The sensor that has the biggest say in how much fuel sprays from the injectors is the MAP sensor. It's a GM-designed part that caused a huge amount of trouble in the first few years they were used, but those original ones should have been replaced many years ago. They generate a signal voltage between 0.5 and 4.5 volts. Anything outside those values will be detected by the Engine Computer which will set a related fault code in memory and turn on the Check Engine light. However, the voltage can be wrong but it won't set a code as long as the voltage is within the acceptable limits. An incorrect value will result in an incorrect amount of fuel entering the engine. Usually MAP sensors fail over a very short time period. They rarely take more than a few days to set a code. More commonly, they fail completely between two drive cycles. When they do fail, one common clue is the engine will stay running as long as the gas pedal is in motion. Position, direction of movement, and speed of movement don't matter. If you release the pedal completely, the engine will stall.

There have been a few cases of similar problems being solved with a new crankshaft position sensor. I don't like throwing parts at a problem without a reasonable explanation of how it can be involved, but I can't argue with the results. There have also been cases of flex plates cracking around the crankshaft bolts. Running problems are caused when the flex plate turns a little which affcts ignition timing. Often a banging sound accompanies the running problem. This is more common on front wheel drive vehicles.

The next plan of attack is to connect a hand-held computer, called a scanner, to view live sensor data during a test drive. Most scanners have a record / playback feature that will record a few seconds of data when the button is pressed. Because the data is stored temporarily as it passes though the scanner, the recording actually starts a few seconds before the button is pressed. The recording, which is taken when the problem occurs, can be viewed later, frame-by-frame to see what the Engine Computer is seeing. Input sensors affect how much fuel enters the engine. The oxygen sensor provides an indication of what happened inside the engine.

caradiodoc

Thank you for your suggestions and obviously you put a lot into trying to give a good answer. First, Idid disconnect the one inch line from the Catilitic converter and it seems there is some exhaust coming through, but it did not make any difference. Believe me that I tried because it does" feel like" something is clogged and the air cannot escape, or it is getting suddenly too much air and backfiring. I tried all of the little air hoses to no avail.
Second, I disconnected the vacuum line to the fuel regulator and plugged the regulator, over and over, but that did not help either. Third, I decided that I had nothing much left to loose so I disconnected the vacuum line to the MAP sensor. Well, that got a reaction!! When I disconnected the line the MAP sensor the engine would start and go to a rough idle, then a squealing sound such as a loose fan belt would come from the front of the engine and then the engine would die. I repeated these three times and all the same. The squealing sound was a power steering belt and I blew a lot of power steering fluid out of an already leaking power steering pump. I do not understand how that relates, but that is what happened. I was going to go purchase a new MAP sensor, but they are over a hundred dollars so I thought I would check with you first. Please let me know if you think a new MAP sensor will fix it. I could by one and if it doesn't work then take it back and trade it for an oxygen sensor? If none of this information helps then I could take it to a shop to see if they will give me a diagnostic. Thank you and yes I do check my mail at night.

Thank you for your appreciation. I just typed a nice long reply, then got the miserable "login" page and everything got lost! GRRR!

So, . . . to start all over again:

Head to a salvage yard for a used MAP sensor. Since GM redesigned them in the early '90s, they haven't caused too much trouble so you shouldn't have any trouble finding a good one. (GM sold them to Chrysler).

To clarify, when you unplug the vacuum hose from the regulator plug the hose to prevent a vacuum leak, not the regulator port. Unplugging the regulator will cause it to increase fuel pressure a little.

My gutt feeling is still leaning toward a plugged catalytic converter. When that happens, the engine will usually idle uncommonly smoothly and you might hear a hiss from the tail pipe instead of the normal "putt putt".

You might want to consider performing a compression test and a cylinder leakage test. I can explain the cylinder leakage test if you've never heard of it. Basically it involves forcing compressed air into each cylinder, one at a time, then looking where the air is leaking out. It will identify, among other things, a leaking intake valve. A leaking intake valve will cause popping through the intake system. This test requires special equipment and a means, usually a little whistle, to determine when the piston is at top dead center on the compression stroke.

caradiodoc

Thank you again.
I did take the hose off of the fuel regulator and put my finger over the hose and nothing noticeable occurred.
You reminded me that when I start the engine that about 4/5 seconds later there is a somewhat loud hissing noise that comes out of the TBI. It sounds like air is whistling or struggling to get by something. I do not think it is the TBI causing it because I have unplugged what I can and it still does it, even when I press down the gas peddle.
So, unless the hissing sound gives you the silver bullet symptom, my last question is what test do you recommend that I have done? After my next payday I can take the van to a shop and get the test done, then have the right information for you.
In the meantime, I will work on the MAP, Oxygen sensor and other things.

As with a snake, a hiss is not a good thing. Look at the throttle blade bushings. If they are worn, they will allow air in. Also, when the engine is still cold, use a spray bottle to squirt water on the vacuum hoses and intake manifold gaskets. If there is a leak, the water will get sucked in and the engine should slow down a little. The mixture will be lean if there is a vacuum leak, but unless you tell me otherwise, I don't think a leak will cause the symptoms you described.

caradiodoc

Hello,
I apologize for my lack of clarity because I should have been more specific about what I was asking in my previous question. What I meant by a test is what type of test should I have a shop perform in order to identify the cause of the backfire. For example, if I go to my local Quality Tune Up shop, what type of test do you recommend would be the best for figuring out what is causing the backfire? Should I have the exhaust? compression? valves? codes read? or what would you test? I imagine a test will run me between 50-100 dollars so I want to spend my money to get information that will identify the cause, or greatly eliminate what is not doing it. This backfire has been going on for about 10 months or more and everybody is having trouble figuring it out. So I will just take it to a shop, but I do not know enough to request what I need. A shop may charge me 80 dollars for a test that I do not need.
>If there is not a good test that a shop can run to figure it out, then I will just go with what you think about the exhaust because that has been my impression as well. I have no idea how to check it, but I will just drill a hole in the exhaust pipe and if the problem goes away then I will know what it is.
For anyone else reading this post I want to say that 2carpros does give a great amount of feedback for the amount of money that I donated.

Don't drill your own hole in the exhaust. The test involves drilling just a tiny 1/8" hole, then the probe of the back pressure gauge is inserted to measure back pressure. That is one test to ask for. To do something similar yourself would require unbolting the exhaust pipe from the exhaust manifold, (if the bolts aren't rusty and ready to snap off), then roaring down the highway to see if performance improves. There might be an improvement if you remove the oxygen sensor. The 3/4" hole will relieve some back pressure, and that might make enough difference to be noticeable.

The next test is called a "cylinder leakage test". It takes a lot longer than a compression test, but is much more informative. One at a time, each piston is brought to top dead center, then compressed air, regulated through the tester, is introduced through the spark plug hole. The mechanical condition of the engine is assessed by observing the percent of leakage and where the air leaks out. Hissing at the tail pipe means the exhaust valve is leaking. Hissing at the oil fill cap or dipstick tube indicates leaking piston rings. Bubbling in the radiator indicates a cracked head, or more commonly, a leaking head gasket. The more likely suspect here would be a leaking intake valve. That would show up as hissing through the throttle body assembly.

Those two tests would identify exhaust gas that is getting into the intake system, either from leakage or pressure. The third test would identify the fuel / air mixture being fired at the wrong time, when the intake valve might still be partially open. That involves watching the spark lines on an engine analyzer. There should be one nice spike for each cylinder when a spark plug fires. If that's the case, but they move around a lot, that would cause erratic ignition timing. The most typical cause of that is worn distributor shaft bushings, but that was more common in Dakotas. Also, it tended to cause surging, not back firing. Of more interest would be any additional, possibly smaller spikes between the regular firing pulses. Those would indicate a stray signal is triggering the Engine Computer to fire the ignition coil extra times. If a spark plug fires when the intake valve is still open, you will hear and feel the back fire. The sensors that deliver the trigger pulses at the exact moment are magnetically operated. Some type of trigger wheel disturbs the magnetic field around the sensor causing the signal voltage to be developed. Anything else that causes a similar signal to occur in that wire will be interpreted by the computer as a trigger pulse, and it will fire the ignition coil. A cracked sensor core results in two smaller, weaker magnets. It still might be able to make a big enough signal for the engine to run, but vibration between the two pieces of the core can generate false extra pulses. Extra pulses can also be "induced" magnetically in the signal wire by "RFI". That's radio frequency interference. When current flows through a wire, a magnetic field is set up around that wire. That is how electromagnets work. When a wire is placed in a magnetic field, it causes a voltage to be developed in that wire. Voltage pulses introduced that way can produce extra triggering pulses to the computer. The most common cause of this is misrouted spark plug wires. They have real high current flow for just a fraction of a second when the spark plug fires. That results in a real high magnetic field that can induce spikes in other wires. (That magnetic field is how inductive-pickup timing lights are triggered). To prevent these stray signals, wire harnesses are specifically routed away from spark plug wires and other sources of magnetic interference. Many engines use metal sleeves over the spark plug wire boots to contain the magnetic field. If extra pulses are seen on the engine analyzer firing the ignition coil, the next step would be to use an oscilloscope function to view the signals coming to the computer to see if stray, extra signals are causing the problem.

Another clue might be gained by momentarily shorting one spark plug wire to ground while the engine is running. If one cylinder always eliminates the back firing, suspect a valve problem or something else related to that single cylinder. To see if just that one plug wire is inducing a pulse into another wire, it must be grounded at the distributor. If it is grounded at the spark plug, current will still flow through it to your ground and the magnetic pulse will still be developed. If there is no significant change when any one plug wire is grounded, the cause of the back firing is related to something all of the cylinders have in common, such as the plugged exhaust.

If the spark lines appear steady and correct on the engine analyzer, a different approach would be to temporarily forget about the back firing. Treat it as a secondary symptom and concentrate on the bogging and low power. Besides the fact the bad stuff must be able to be freely pushed through the exhaust system, the good stuff must be made up of the proper ratio of fuel and air, and it must be burned at the proper time. An incorrect amount of fuel or air to one cylinder only will result in a misfire you can feel but the rest of the cylinders will provide plenty of power. When hesitation or bogging down are involved, all the cylinders are responsible. They are all not getting enough fuel or enough air. With two injectors instead of one for each cylinder, a partially plugged injector will result in an excessively lean mixture to half of the cylinders. Since there isn't much in the way of incoming air, a fuel problem is more likely. Now the question becomes whether the injectors aren't delivering as much fuel as the computer is requesting, or if the computer isn't requesting enough fuel in response to incorrect sensor data. This is where the experience of a driveability mechanic comes in. When a sensor sends an obviously wrong signal, the computer detects it, sets a diagnostic code in memory, and turns on the Check Engine light. When the sensor's signal is within the realm of possibility, but it doesn't agree with other sensors or the operating characteristics of the engine, a different fault code could be set. An example would be the MAP sensor indicating a real heavy load on the engine while the throttle position sensor says we're at idle. The third type of sensor failure involves a believable but incorrect value. These are the ones that don't trigger a fault code so the mechanic has to evaluate what the Engine Computer is seeing and determine if those readings are correct or are causing the problem.

So there's a bunch of ideas. All major cities have specialty shops that concentrate on these types of problems. They might be less expensive in the long run if they are already familiar with looking for these things. If you are near Jolliet, IL, I can find the name of one such shop. One of the fellows there is a Carquest trainer who travels to a few different states to put on relatively high-level classes. They have come up with some really unusual causes of problems and many unique ways to quickly find them.

Please forgive me when I take so long to reply. My Verizon e-mail is going through major changes and they are out-of-service about 20 hours per day for the last couple of weeks. Normally I receive an automated message when you post a followup response. Now I might not see that message for many hours, and the direct link to it doesn't work either.

caradiodoc

Hello,
Thank you for the information that I asked for. I will print what you have written and take it with me. It will be 2 - 3 weeks until the tests are done. I will email you when I have the results so that we can begin with donation and thread.
Again, thank you.

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