Here is a list of what to check if your car is backfiring constantly or if the car is not running right.
Ignition box (CDI/CDS)
First thing to check is that the ignition box is making squeling sound while power is on, this indicates that the high power voltage is produced.
If there is no sound check that the ignition box is getting correct voltage (Check the manual which terminal) If the voltage is much under 12V the ignition box will not sound "healthy". If you have heard the sound while its working, you know the difference.
High voltage terminal should be reading 4-5V, and when measured with oscilloscope there should be this 400-500V spike when terminal C is connected to ground.
Ignition coil
Ignition coil is simple to measure. When measured with ohm meter the primary coil windings should give reading of 0.3-0.6 ohms. Secondary coils should be near 600 ohms.
Check that the wires that are attached to coil are correctly connected
Never use conventional ignition coil with CDI/CDS system
Ignition wires/connectors
Check the wires with an ohmmeter that they all conduct. Copper wires should give very low readings, refer manufacturer information on other types of wires
Wires could leak the "spark" to ground therefore diminishing the spark that goes to spark plugs. Putting tape on the wire to prevent leak won’t help. Replace the wires, they are cheap.
Resistors (plug/distributor caps) need to be measured, they should all be in +/- 10% ohm range of each others (2k ohm on each end is normal)
Resistors are not needed, but dismissing of those may cause radio interference in stereo equipment or other digital instruments.
Spark plugs
Check the plug condition visually and clean the plugs with brass brush.
Check the gap between the electrodes (should be 0,6mm or near)
Plug insulator may be cracked and thus "spark" is leaking to ground.
Breaker points
Check visually that the breaker points open enough (refer to manual) when manually turning the engine (BE SURE THAT THE IGNITION IS OFF!!)
The breaker point contacts should be clean and even. If there are any spikes or holes with on the contacts, replaec the points with new ones.
Check with "light pen" that the breaker points open at right position
Breaker point gap is directly connected to dwell angle. To be sure that the gap is correctly set, measure the dwell angle with correct meter (many multimeters have this nowadays).
Vacuum advance/retard
While distributor cap is off you can check if the vacuum advance system is working
Remove the hose from the carburetor and suck it (with your mouth). The breaker points should be moving a bit
If nothing happens the breaker point advance/retard system is stuck. Lubrication usually helps on this one.
If the breaker points are moving just fine when moving with hand, the problem could be the vacuum unit. When aging these usually break and should be replaced.
Carburetor
The carburetor could be getting dirty and clogged so the mixture is not right, if possible check the jets and emulsion tubes that they are clean. With weber carbs the idle and main jets are easily removable, they are at the opposite side of acceleration pump. Even if these look clean, clean them with compressed air. Don’t poke them with anything metallic, the jets might get damaged and the jet sizing is not correct anymore. If the jets are very much covered in oxides you could soak them in some solvent overnight or try cleaning them ultrasonic washer.
Air leaks can cause backfiring, the leaks cause that the engine is getting too much air and so the mixture is not correct. You can either install new gaskets to carburetor or use some gasket adhesive to correct the situation. Don’t use any silicone based substance because they can’t stand the heat+gasoline combination, even though the markings on the side of the box indicate so. Besides, when you put the carburetor back together some silicone adhesive could get instantly in carburetor and from there to the jets and emulsion tubes.
The gasket between the engine and the intake manifold as the gasket between carburetor and intake manifold might be leaking. These are usually made of graphite or other material and need to be replaced with new ones.
The fuel mixture may be too lean and cause backfiring and uneven run.
Comments and Suggestions:
Fastco
Comments: I have a 1982 911SC. The car will stert cold fine and run anywhere from 20 miles to 200 miles. Then for no apparent reasom starts to quit and backfire. It will usually idle but runs rough.If you turn the ignition switch off and restart the car it will run fine untill it does it again. I have replaced the fuel pump relay, fuel pump and check valve. all spark plugs ,wires,rotor,cap,fuel filter ect. Can this be a bad ground?look where CDI box or ICU? Help
April 25, 2012
Jordan2550
Comments: 1983 Porsche 944- I have an intermittent start that seems to have no rhyme or reason as to when it decides not to start. It will crank just fine, but no start, and then ill go back an hour later without doing a thing and it will start right up. One of the times it didnt start, i found it had no spark, and so i traced the wires with he electrical tester and found the coil pack had power, but nothing was going from the distributer. could coil pacs start to have intermittent problems before they go? also, after i start it it doesnt idle well untill it warms up a bit.
I was also told it could be a vaccuum leak, but if so, from where?
PLEASE HELP!
April 4, 2012
matt
Comments: i have a 83 porsche 944 i was driving the car every day the it would not start it was one of the batter wires positive side so i connect but now the we NOT turn off unless i stole out and now the light wont go on just the head light the tail light go on if anyone knows what this problem is please help
March 6, 2012
BernieF
Comments: Hi, I'm a proud owner of a 1970 911T. The car has worked perfectly for the entire 5 years I've owned it, but I now seem to be getting constant battery problems. The car runs on one battery only, currently a Bosch Silver S3 12V 53Ah. We have changed the alternator but the problem hasn't gone. Should I look for a stronger battery? Any recommendations? Many thanks, Bernardo
February 23, 2012
Jon
Comments: 1998 boxster triptronic.
I had the tranny rebuilt. It upshifts fine. Downshifts properly sometimes. Most of the time it does not downshift till I come to a complete stop, then it downshifts hard. Sometimes when I am slowing to a stop, I can see that it goes from 5 to 4, then back to 5 gear. Every now and then it would upshift adn downshift properly. Any suggestions??
January 31, 2012
porscheonebit
Comments: I own a 78 Porsche 911SC. The starter continues to run, even with the ignition switch off, I must disconnect the battery at the terminal to shut it down. I looking for information on where I begin to look to correct this problem. HELP
January 9, 2012
matt
Comments: I have a 1997 Boxster with a 2.5L the drivers side cylinders 4-6 have no spark but the passenger side cylinders 1-3 does. The car will actually start and run but as you can image its rough. Any help?
Thanks Matt
November 28, 2011
Followup from the Pelican Staff: Hmm, I'm a bit puzzled by this. I'm curious as to how you know there is no spark on 4-6? I don't know of any way to actually test this, as the coils are on the plugs. I suppose you could take the plug / coil out and see if you can see a spark, but I'm not sure the wire harness would be long enough for that. I suspect that you actually mean that you have no combustion on that side, and you can tell that from the headers not getting hot. Seems like you need to plug the car into a code reader and see what it will tell you - that will give you some big clues on this. - Wayne at Pelican Parts
yogi
Comments: have had an early 1985 944 for a couple of years - no problems easy start and runs like a dream. all of a sudden it died on me, cranked it a few times it started a little irregular a block later dead. towed to mechanic who inferred that it need the two crank sensors i bought one from pelican and another from another supplier I ALSO READ CLARKS GARAGE THAT THE DME COULD HAVE BEEN RESPONSIBLE and so i got one of those. everything installed - car won't start. it had started with old sensors but only ran for an hour and quit and no more start. after new stuff installed and car not start mechanic claims sensors again, but no start and cant use old ones cause one broke in removal. thgis mechanic is usually pretty good but not a porsche man and i think he is lost as to what the problem is. any ideas for me???? eric
October 13, 2011
Night Train
Comments: I have a 1986 928S and I am trying to hook up the vacuum system, does anyone have a picture or schematic of the vacuum system? Thanks Bobby
September 7, 2011
horse292
Comments: I recently purchased a 74 914 with a 2.0 that was basically in pieces
the engine was "professionally rebuilt" but after I finally got every thing together and started it has a dead #3 and 4 cylinders.
the Basics
Pertronix ign, Empi carb setup // I set the timing for carbs34 at 3200RPM and tried to balance the carbs but couldn't get the idle right, started pulling plug wires and 3&4 made no difference. Last night I was going to adjust the valves and set it all up according to the articles I've read but At TDC my #1 cylinder rockers are tight and #3 are loose if I turn the crank so the mark aligns on the bottom #2 adjustment #1 is loose. I'm thinking it was assembled 180 out. I've only rebuilt American v-8s and I've never been inside one of these but I'm assuming this basic error can be made assembling it as well. My question is what now?
September 7, 2011
kiteboardingjones
Comments: @Racecar80 - I'm seeing the same thing on my 914-4 with single carb. I think my car is only running on #1 and #2. The plugs are fouled only on #3 and #4 so I'm pretty sure they are not firing because I also pulled the wires and saw no change in the way the engine was running. I verified the value adjustment and they are perfect. I swapped the plugs and verified the gap but no change. Car is timed correctly. Now at a loss as to what to check next.
August 21, 2011
Greg
Comments: My 82 911sc misses and backfires until it is driven for about 20 minutes then it runs well. I replaced the ignition box, coil, injectors, wires, and plugs. No effect. Any ideas what could be left?
July 30, 2011
werner
Comments: I was driving my 1987 porsche turbo and it began to stater and finally turned off. it seams that it has no fuel and no spark. any idea what went wrong?
July 25, 2011
mikeb
Comments: My 914 will turn over and fire with starter fluid and then it shuts right back off. Any ideas of what the problem is?
Thanks
July 5, 2011
GDS
Comments: My 1972 914 has decided not to start. In checking the vacuum unit on the distributor, "Remove the hose from the carburetor and suck it with your mouth. The breaker points should be moving a bit" -I have no movement of the points. I wondering is there any way to get the car started even though the vacuum advance may be faulty?
July 2, 2011
Spike
Comments: I just installed a 1984 3.0 engine into my 78'SC body, it seems that the mating of the wiring is correct and got it to start and run rough a couple of times, now it won't start. I am getting power from the DME to the coil but no spark. Where do I start?
June 28, 2011
Racecar80
Comments: I have a '74 914 2.0 & just did a complete rebuild w parts from pelican; new cam, pistons & jugs, rings etc, etc. The engine has dual weber 34's on it. It starts great and after it warms up will idle pretty well a little rough maybe, it rev's up strong with no load In Garage, but is a dog when you try to accelerate driving. I smoke tested it to look for vacume leaks ok. But i notice that when i pull #1 &/or #3 plugwires off the distributor cap, there's no change at idle. Compression is even on all cylinders 110psi & spark is good too. Anyone have any suggestions?
June 10, 2011
Huh
Comments: Have a 1981 Euro SC that will not start. Even when I spray starter fluid in the air box, it will not fire. The engine turns over, I've replaced the ignition coil with a known good used one. Usually I would push up on the fuel distributor plate, then it would start. When measuring across the ignition coil, it should be what is stated above? What is the secondary coil?
April 24, 2011
Jim
Comments: i have a 1976 911s. Accelerates well but as soon as i back off on the gas, she produces a gun shot style backfire. I have checked exhaust system for leaks....none. I am scaring the heck outa people!!
April 22, 2011
Bandit
Comments: I have a 1976 Porsche 914 2.0 Starts, runs for about ten seconds and quits, I am not getting any power to the fuel injectors while it cranks and runs, can anyone help me I need a specific procedure for checking the inlet pressure sensor either with an ohmeter o with voltage I don't own a sophisticated tester. Thanks, Jaime
April 18, 2011
Followup from the Pelican Staff: Well, the old way that we used to test to see if they were any good was to just suck on them. If you could suck through them, then they were bad. If they held a vacuum, then they were good. Not scientific, but very useful for checking the units when we found them at the local swapmeets. - Wayne at Pelican Parts
AL
Comments: I have a 1989 turbo. Problem is the engine will cut out without warning. This can happen while driving or idleing. I think it's the cdi unit. They make 2 types for this car. I would like to replace but cannot find any.
April 12, 2011
Followup from the Pelican Staff: We can rebuild these if you'd like. I will forward your email address to our sales department and they can fill you in on all the details. - Wayne at Pelican Parts
Bandit
Comments: I have a 1976 Porsche 914 2.0 It turns over real good I have a good spark but fails to start, any help? Thanks
March 29, 2011
Followup from the Pelican Staff: To start the car, you need compression, fuel, spark and proper timing. Guaranteed, it's one of those. I would probably check the timing first - I've had issues with my 914-4 in the past not starting when I messed with the timing in the distributor. - Wayne at Pelican Parts
Troy
Comments: I have a 1982 911sc. Car will run great, but at times, it will just cut out - for no reason whatsoever. Just stops running. Sometimes I can jump start while running and most others, I will have to wait up to 30 minutes until it will start again. This problem has become more frequent as time passes. I have replaced the starter and have had the fuel system looked at. Do you have any thoughts? Thank You.
Comments: I have a 70 914 with a 1.7.. It is stumbling on acceleration. It had a 009 dist. I changed it to a Mallory and Mallory coil. It is a little better but still stumbling. I have EMPI carb, single barrel, not sure of the size, came with the car. Am I under carbed, jet combo info. I could use some suggestions. Oh ya, idles fine and accel is fine without a load.
January 19, 2011
Followup from the Pelican Staff: I'm guessing indeed that your jetting is way off. I would use a handheld O2 meter to check the exhaust and see if you're going rich or lean on acceleration, and then that will guide you on what changes you need to make to the carb jetting. - Wayne at Pelican Parts
Chronictinkerer
Comments: I have a 1980 911SC. Cranks but won't fire when hot. After 2 hrs sitting with hood open it will start. Changed the coil, no change. Definitely a hot soak problem. What else can I try?
Thank you.
January 17, 2011
Followup from the Pelican Staff: Open up the air filter and reach under there and push up on the sensor plate that is in there. You should hear the injectors make a "meow" sound. See if it starts after that. Sounds like a hot start problem that is typical of CIS systems. You need to either replace the fuel accumulator or the check valve in the fuel pump (which requires replacing the pump). - Wayne at Pelican Parts
charles
Comments: I have a 1986 944 turbo, 66,000 original miles, took to shop to get oil leak fixed, and it started missing on the highway at any level of speed, new wires, new rotor, new fuel pump, new filter, new tps sensor, now they want to replace maf $1000, tested and it has no problem with the volt meter, car does not miss under acceleration or deceleration, only at level speed in any gear can be at 30, 40, all the way up to 100, but once again only misses on a level plane, can anybody help?? thanks charles 325-518-6073
October 14, 2010
Followup from the Pelican Staff: I would start by getting one of those meters that allows you to monitor the mixture. If it's really rich or lean, then you might get missing, and that might be an indicator of whether or not the MAF is going out. If you've replaced everything on the ignition, then I doubt it's a problem there. Or, swap out the MAF with a friend's one that works to see if that solves the problem. - Wayne at Pelican Parts
ken
Comments: I have a 1985 944,just stated working on,other people tryed to fix it,new air flow sensor,pressure regulator,pressure damper. I put new spark plugs,cleaned injectors, it now trys to start but floods out,If pull the rail out and turn it over it runs till the gas out, and the injectors are a spraying while it is runing,i ordered a throttle switch,I have not checked the pressure on the rail yet,am I on the right track? ken
October 11, 2010
toyboy
Comments: I have installed a new engine in my 1985 944 the starter is trying to engauge but wont, if i bench test it it is ok. The engine turns easy, there is a wire coming off of the battery that appears to rout to starter going around booster however if I hook this wire to starter then it will engauge when battery ic conected. cant figure out where this wire goes any advise
August 24, 2010
Followup from the Pelican Staff: There should be two powered wires connected to the starter. One goes directly to the battery and powers the motor. The other comes from the ignition switch area and powers the solenoid. When the solenoid is engaged, then it automatically applies power to the starter motor. So, if you hook the cable from the battery to the solenoid terminal, the starter will engage automatically when you connect the battery. Bottomline, connect to the other terminal. - Wayne at Pelican Parts
gb
Comments: I installed a slow trickle charger on my 914-4. Went to start the car and it would not start. Did run 24 hrs ago with no problem and engine started. Radio and all electronics went on, but when I turned the key the starter just whined at a high pitch. Engine did not turn over. Could it be that that starter is getting too much juice?
GB
August 23, 2010
Followup from the Pelican Staff: Could be one of many things:
- Igntion switch (replace)
- Old, worn out wiring with high resistance (add hot start relay)
- Missing or loose ground strap (reconnect)
- Weak battery (strong enough to power radio, but not starter?)
Try putting a big charger on the battery and seeing if that helps. I would probably start too by replacing the battery or at least having it tested.
- Wayne at Pelican Parts
729146
Comments: I have a 72 914-6 and just installed the engine after the car was not running for 30 years but am not getting any spark what do i need to look for cdi is working but no power to the coil.
Joe
August 13, 2010
Followup from the Pelican Staff: I think you just need to go through and check all the connections. There are fuses in the back too on the relay board that can be loose, as well as relays that can have corroded contacts. The relay board itself can get old and corroded internally too. - Wayne at Pelican Parts
David
Comments: I have installed a Pertronix Ignitor and coil in my 74 911. I have triple checked the wiring and tested components but it still will not start. At first I wired it wrong but have tested the unit, the coil and the CD and all test good. Do you have any suggestions. I'm going crazy!
April 13, 2010
Dave
Comments: Hi,
I have a 914 Porsche, year 1974. I'd like to replace the ignition lock cylinder. Can anyone help me find my way through it? Thanks in advance. Dave
March 23, 2010
Dave
Comments: Hi,
I have a 914 Porsche, year 1974. I'd like to replace the ignition lock cylinder. Can anyone help me find my way through it. Thanks in advance. Dave
March 23, 2010
Barry
Comments: I have a 968 that is running rough up to about 1300 rpm, and the "EX" warning light comes on from time to time.
The manual has NO mention of what this EX light tells me. Does anybody know what it means?
Any ideas ?
February 13, 2010
Sean
Comments: Hi there,
I am trying to find a Magneti Marelli breaker point plate/rotar arm from a Marelli distributor. I own a 1970 911T, which will not fire up. I have attached a picture any idea??
thanks
Sean
February 10, 2010
cp
Comments: do you have a schematic & thead for a 6 point cdi box or any more information for a 1983 911sc.
January 9, 2010
MING
Comments: IS THERE A PROBLEM IF I USE PORSCHE 944 IGNITION COILS IN MY 928 PORSCHE ?
October 16, 2009
Followup from the Pelican Staff: I'm not sure, but I don't think so. The 928 is basically two 944 engines put together. It's probably worth a try, and I doubt you would blow anything up. - Wayne at Pelican Parts
XNissanTech
Comments: This era of Nissan is notorious for faulty STARTER RELAYS - most all models in Nissan's lineup are prone to having this failure. Have someone turn the key to the "start" position while you are touching the relay - if it does not "click", the relay is most likely the problem.
September 18, 2009
Followup from the Pelican Staff: Indeed, my 1988 Pathfinder once had this problem happen to it! - Wayne at Pelican Parts
isaac
Comments: my Nissan Maxima 96 3000cc Wont turn on sometimes. It happens only when the engine is warm.I changed fuel pump new battery and new srarter, please help.
July 19, 2009
Followup from the Pelican Staff: Hot start problems can be caused by many different things, but usually it's the starter. When an old, tired starter gets hot, it can sometimes stop working. If it were my car, I would probably replace the starter with a new unit first. - Wayne at Pelican Parts
