1998 Chev K2500 - 7.4L Miss Under Load FIXED!!!!

[protraxrptr17]

Another shop installed all kinds of new ignition parts and a TPS. I ran a leakdown and found leaky valves. Put it all back together and problem is still there.

Now it throws a P1336 CKP variation not learned. I scoped the crank sensor and the signature looks OK. Ran a frequency test and get a bad glitch during the miss. Could the glitch be from the miss, or is it the cause of the problem? I swapped sensors from another engine (I know that really doesnt mean anything) with no change. I took a quick look at the tone wheel while the sensor was out. Didnt see anything major.

I will see if I can post a screenshot.

[steven kiser]

this is a prime example (in my opinion) where someone else may have put a plug in thats not gapped correctly. these issues i preach yank all that was installed and put my parts in.

[nickscarcare]

Did you do a crank relearn on this at all. I have seen where these loose the learn on these and doing a crank relearn fixes the miss. Guessing that if you fixed the leaking valves, you did look at all the plugs to make sure of proper gap and proper application, and checked for cracked plugs.

[protraxrptr17]

I forgot to add that I cant get it to take a crank relearn. Tried it numerous times. Plugs all look nice and new. It was doing the same thing before and after the other shop worked on it.

[ricmorin]

Check the distributor. I've seen a few of these with loose bushings.

[protraxrptr17]

maybe this will work

[protraxrptr17]

Ha!! It worked. Love the Verus.

[Tim Martin]

As I understand the ckp and cmp sensors, the crankshaft sensor simply tells the PCM that the crankshaft is rotating and where #1 is. The camshaft is used for fine tuning spark and injector firing. The glitch you see in that scope is from the miss. I am thinking that the ckp sensor will not create a miss, although I suppose anything is possible.

[ricmorin]

You are running a frequency test. Any engine speed fluctuation will show here. Better to do a signature test and look at the actual waveform. You can get a misfire when the cam/crank is not learned.

[ricmorin]

Also, on some rare occasions I've seen a worn timing chain cause this, but usually it's the gear at the end of the distributor, worn distributor or a distributor that is not installed properly.

[ricmorin]

As I understand the ckp and cmp sensors, the crankshaft sensor simply tells the PCM that the crankshaft is rotating and where #1 is. The camshaft is used for fine tuning spark and injector firing. The glitch you see in that scope is from the miss. I am thinking that the ckp sensor will not create a miss, although I suppose anything is possible.

You may be mistaken. Since the crank sensor cannot know if it's on the compression TDC or the exhaust TDC, it cannot be the determinor for TDC. The cam sensor determines TDC. It provides one pulse per camshaft revolution. When cam sensors go out, the engine often cranks longer because it cannot sort out where TDC is.

[liljoe]

As I understand the ckp and cmp sensors, the crankshaft sensor simply tells the PCM that the crankshaft is rotating and where #1 is. The camshaft is used for fine tuning spark and injector firing. The glitch you see in that scope is from the miss. I am thinking that the ckp sensor will not create a miss, although I suppose anything is possible.

You may be mistaken. Since the crank sensor cannot know if it's on the compression TDC or the exhaust TDC, it cannot be the determinor for TDC. The cam sensor determines TDC. It provides one pulse per camshaft revolution. When cam sensors go out, the engine often cranks longer because it cannot sort out where TDC is.

Actually, it will knowTDC, just not weither it is compression or exhaust stroke, missing CMP will cause fuel injection to become batch fire on most modles that will run with out one. I agree that the problem probley lies in the dist gear or timing chain/gears.....

[protraxrptr17]

OK, double checked the dist gear (looks new). Found the "new" tps was shorted causing the ref volts to be low. Fixed that, but no change. Cant get that ckp glitch to show up again, so I guess that was a freak thing. Still wont crank variation learn. Gotta be related to the miss.

[protraxrptr17]

Timing chain has some slack. Not sure if its enough to cause a problem but I guess we will see. Can that cause the no learn situation? Seems like I keep finding problems but none help the symptoms. Makes me look like I'm guessing at stuff, but I can easily put my hands or eyes on all these problems.

[Tim Martin]

Ya know what? It wouldn't surprise me if the 1336 code and the miss are not related. So far as the miss is concerned, you could have an injector that is failing under load. Has that been checked?

Also, what is the camshaft retard degrees at 1000 rpm? This could be a factor in this issue. It should be plus or minus 2degrees

[steven kiser]

i've had a few times where it's been a bear setting the distributor. if i'm thinking of the correct situation you need to be set dead nuts on 0, set the distributor by the color dot on the gear and drop it in. i usually put a quarter against the intake and push the distributor (with the cap on it) against it. this is the closest manual way i've found. i have run into a few after market units that walk way to much in the intake port. every time i would read it the reading would vary. again, i'll repeat my feelings of sweeping up others messes. since they've been taken out of the equation by you working on it i always remove any parts installed by other shops. your laying you reputation on the line. years ago i replaced a cracked cap and it returned numerous times for a bad plug, leaking fuel filter, poor fuel mileage. all because i straightened out the initial issue all others were my responsibility. as far as the customer was concerned he paid me to resolve a driveability issue and it wasn't resolved. from that day on the only way i'll work on a vehicle that has bounced from shop to shop is to box up all parts that were installed prior to coming to my shop and install mine. i also inspect the vehicle for any mechanical fubar issues before i work on it. people, after spending a lot of money on a vehicle tend to look for a * to beat up. it's usually the last shop that works on it. i had one person screaming at me because i found a broken wire that was causing all his issues after he had spent over a grand at other shops that didn't resolve the issue.

[protraxrptr17]

I current ramp tested all the injectors. When setting the retard I did have trouble getting it to settle on 0. Always wanted to go to -1 or +1. I think that was due to the timing chain slack.

[Tim Martin]

Will current ramping reveal inadequate fuel flow through the injector? I have had injectors in the past that electrically tested fine but were unable to inject sufficient fuel when needed.

[Tim Martin]

Oh yes, I forgot to mention, so far as setting the retard degrees, based on the numbers you have, you should be fine there.

Also, have you examined the inside of the distributor cap for current arcing under load? They were notorious for that.

[protraxrptr17]

I do need to look at the cap better. Its new, but....

I dont think current ramping will show a fuel supply issue. I ran a injector balance test also. It seems as if it runs rich going by the O2's during the misfirin Making me think its a timing issue. Timing chain supposed to be here tomorrow morning. Then we will see whats next. I dont have much confidence in the chain.