Misfire | 1999 GMC Safari Van | 4.3
My first case study is on a 1999 GMC Safari van with a 4.3 liter. This was the first job Dewitz Diagnostic Solutions did for this new business account. The complaint was a bad misfire on cylinder #2 with some other random misses on the other cylinders. This vehicle had a poor maintenance record so the shop did a basic tune-up to get the ball rolling.
The van still had the miss when hot and was also setting a P01345 cam crank correlation error code along with the basic P0300 and P0302. After they tried a used distributor (cam sensor) with no change and ran out of ideas they gave me a call. I came to there shop (one of the cleanest in town) and took a look at the GMC. I also pulled the same codes they had and he also showed me that when it starts to misfire, you can hear the spark snapping inside the distributor cap. While watching the cam retard on my Tech II, I saw it jumping all over the place from -8 degrees to -21 degrees. What could cause this?
The cam retard should be holding steady at around -10 degrees at 1000 rpm. Well a bad cam sensor, timing chain, distributor gear, crank sensor, excess crankshaft end play could all cause this. GM’s trouble tree for code P01345 pretty much states that at 1000 rpm you should have between -5 deg. and -15 deg. and if you rev the engine to 2000 rpm there should be no more than a 2 deg. change. If there is more than a 2 deg. change, replace the distributor (cam sensor). So I scoped the cam and crank signals together to see if they stayed in sync or not. No matter what I did with the engine rpm’s the cam and crank signals did not move more than a degree from each other if that. After some final checks I didn’t really have a conclusive answer except that the VCM could be faulty causing the cam retard to jump all over even though it wasn’t. I told the owner to get a VCM but I let him know to try to get a used one he could return because I wasn’t nearly convinced this was the problem but had nothing else to condemn as bad (and it was 9pm).
His employee stop at the shop so we could hook up the new VCM, program it, relearn the theft system and see if this fixed the issue. The short answer was, I flashed the used VCM for free since it did not cure our issue. Back at his shop and with a new approach I scoped the crank signal at the sensor and at the VCM to make sure we had no voltage drops in our signal wire to the VCM. We didn’t but the problem became very clear to me now. When it would start missing the top of the crank signal would start dropping out a little especially in one spot causing the #2 cyl. misfire.
A bad tooth on the crank could cause one spot to keep dropping out but I could see that it was dropping out in other spots also, just not as consistently. It was a new crank sensor that was faulty. How did I miss this the first night? Both images with the cam and crank sensors where captured that the first night and you can clearly see the problem. Well, I was more interested to see if they were staying in sync with each other than anything else and with all the noise on the cam sensor (upside down primary coil amperage and secondary spark kv) I just figured the same was on the crank signal. Also after using a scope day to day you learn not to over analyze all of your scope patterns because you do get different types of interference that you have to learn to ignore so you don’t replace good parts. What I learned this night was that there is a fine line between over analyzing and under analyzing and sometimes when your stuck on an idea to let it go and try to see it from another perspective.
Leave any questions or comments below. Thank you.
17 comments
I have the same gmc safari 1999 and I am getting the same error the misfire on cylinder 3 I read your above post and was wondering if this is the likely answer
It is possible but the problem above is not very common. I would start with the basics first especially checking the injector poppets.
We are having a similar issue. 1998, misfire, codes say random misfire, but based on pulling plugs off the distributor it is Cyl 1. Just don’t know why – sensors? Injector?… I can do the parts swap, but need to isolate the cause, can’t afford to just throw parts at it and really don’t want to take the intake apart unless I know it is the injector array.
Your next step would to do a injector pressure drop test. That will isolate the problem to the injectors and which one.
Have a 1999 Olds Bravada. it has a #1 and 3 misfire. There are occasional misfire on #2 and 4. there is no problems on #5 and 6. Did the pressure bleed down and found a problem on the spray pattern on #1 and 3. Replaced the spider and the same problem. The fuel pressure is 55 and the filter is good. Compression is steady across the board. I have some good training thru the military. this has me confused. thanks for some help
Random misfires can be hard to track down. Defective distributor caps are common to cause random misfires. When does it misfire? Idle, under load? Valve springs, egr, vacuum leaks, plugs, fuel quality are just a few ideas. Would really need to know more. What are your fuel trims?
The problem is @ an idle and goes away above 1000 rpm. the spider injector had a poor spray pattern on the # 1 and 3. they had the injector replaced and the same problem. compression is good in all cylinders and is 125 to 130. used a scanner and tested the egr by increasing the %. used a vacuum gauge and the vacuum is bout 22. fuel flow is normal. the cap and rotor are new A/C delco. the wires are autolite 7mm. the distributor shaft might b grounding out @ idle.
The problem is @ an idle and goes away above 1000 rpm. the spider injector had a poor spray pattern on the # 1 and 3. they had the injector replaced and the same problem. compression is good in all cylinders and is 125 to 130. used a scanner and tested the egr by increasing the %. used a vacuum gauge and the vacuum is bout 22. fuel flow is normal. the cap and rotor are new A/C delco. the wires are autolite 7mm. the distributor shaft might b grounding out @ idle.
I think there is a TSB on crank sensor spacing causing random misfires. I would also do a running compression test. A difference of 5 psi can be the cause of a misfire on a running compression test. Check this case study about the distributor out also – http://www.dewitzdiagnosticsolutions.com/case-studies/misfire-light-load-2000-2500-rpm-1996-chevrolet-blazer-4-3/
Bad new parts are always a possibility – do you have the ability to scope the ignition system?
No scope. retired marine that helps people that cant afford real help. have had good classes and understand most. the compression test is good running. can the distributor be isolated to certain cylinders? I have the genisys scanner. will the crank sensor isolate only certain cylinders?
The only way to maybe see a distributor issue with just a scan tool would be to monitor the Cam Retard PID at 1000 RPM or higher. Should be steady.
thanks
I have 2000 Astro 4.3 with random misfire P0300 you only notice any issue when trying to maintain highway speed and then you feel the misfire let out of the fuel, add slight fuel, or pulling up a hill and it goes away. three shops have scoped it only consistent signs is random misfire on 6 of 5 cylinders. Occasionally it will throw a P0102 Mass or Volume Air Flow Circuit Low Input however it scopes out fine. The cam retard is @ -12 and only varies by 2. I have tried three used distributors from a friends garage and they showed the same. The misfire shows up worse on humid or rainy days. The distributor cap does collect green corrosion constantly. i have changed fuel pump, filter, cap and bug, wires, plugs, cat, cleaned the maf! Any idea?
A P0102 Mass or Volume Air Flow Circuit Low Input is most likely a bad MAF sensor. Other causes are wiring, TPS, or ECM. Usually when you have that code the MAF either works or doesn’t. When it is working it will test good. The misfire issue may or may not be related. The 4.3 distributors need the screens punched out since those are made for marine use and trap everything in the cap. You may have valve float issues or a number of other problems. More testing is needed.
Hi, Travis I think I need a Crankshaft Sensor too I have the same symptoms with my 98 Astro 4.3 but my computer throws no codes. I have replaced the cap, wires, plugs, coil, ignition module, and fuel pump cause that had only 45 psi. It wants to start but won’t and you can hear the ark in the distributor cap.
Hear the ark? Not a valid test. You need to have a spark tester that can load the system, and start your test at the spark plug and work your way back. But if you do have spark, why would you need a crank sensor for a no start? Finding a shop that you can trust to do actual testing without just replacing parts would be worth its weight in gold in my book. Usually the good parts on your vehicle that have 100k on them are better then what you can buy new in the aftermarket which is why I use only a few trusted manufacturers. A lot of the time I see people almost fix their vehicles only to have defective new parts cause them to replace more. Testing correctly solves this issue.
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