Misfire issues with 98 Yukon

erockwell · May 14, 2018

I'm having a bit of a dilemma here. I have a 98 Yukon and back in January it started missing. I replaced the plugs, wires, cap, and rotor, and it worked fine for a while. A few weeks ago, it started missing again, this time much worse. I replaced the O2 sensors and that didn't fix it, so I took it to the dealership to have it diagnosed. The dealership said that the distributor and the injectors were bad. I replaced the distributor with one from a salvage yard, and upgraded the old spider to a brand new AC Delco MFI injector system. I just finished replacing the injectors yesterday morning. I cleared the codes and took it for a drive. It all seemed fine, but then I noticed a slight hesitation and sure enough it showed the same misfire code. At this point I feel like I've done everything I can.

When I put the intake back together I noticed a few things. One is that one of the screw holes on the new distributor is stripped, and wouldn't tighten down all the way, so I'm wondering if a slightly loose cap could be having an effect, and I'm also wondering if the plugs could have gotten messed up while the engine was missing really badly and if I should replace them again. I didn't replace the MAF senor when I did the O2 sensors, and I'm wondering if I should have, and I'm also wondering if I should take out the salvaged distributor and replace it with a new one. I found a good deal on one through Amazon.

Knowing all that, is there anything else that could be causing this slight hesitation after all the work I've already done?

Thanks in advance,

Eric

bronyaur · May 16, 2018

You need to set the CMP offset. It needs to be 0, but at least +/-2. This involves moving the distributor slightly either direction until you get the CMP offset to 0. I've seen it cause a hesitation without setting a code, and I haven't seen the code come on until its around +/-14. You'll need a scan tool that can read the CMP offset. I use the DashCommand app on my phone with a wireless scanner that plugs into the OBD port.

Ditch that distributor and get a new one. The best I've found is a billet aluminum from Summit Racing (after having nightmare issues with a couple plastic ones from NAPA). The one from the salvage yard might have a worn gear, even if it doesn't look like it. And, with the cap not being securely attached, that cap might just be floating on there. These engines are really picky about the distributor.

Also, you didn't mention changing the fuel filter, but that's a good place to start if you haven't yet. You can get a can of MAF cleaner and take care of that (assuming it's otherwise in working condition), and you can pull the plugs to see if they're fouled. Try running the engine at night in the dark and you may be able to see arcing around the distributor or wires.

erockwell · May 19, 2018

Thanks for the excellent advice! No, I haven’t done the fuel filter yet but that’s a good suggestion and I’m going to that ASAP. I just replaced the spark plugs and several of them were fouled. Although that helped with the hesitation, it didn’t eliminate it entirely. The idle is much smoother and the acceleration is improved, but I don’t think it’s 100% fixed yet. I believe what you said about the distributor seems very likely to be the issue, so I’m going to shop around for a good new one. I really don’t want to break the bank but at the same time I don’t want to get a piece of junk, so I may post some possibilities on here to see what you think.

I looked into the dashcommand app and it looks really cool, but I couldn’t find much info about compatible scanners. I looked on Amazon and found wireless OBD2 scanners ranging in price from 10-300 bucks, and once again while I don’t want to get a cheap one I don’t want to pay a premium either. If you know of any decent ones for between 50 and 100 bucks, that’s what I’m prepared to spend on a scanner.

Thanks again for the advice.

EDIT:

I found a Bluetooth enabled scanner on Amazon that’s compatible with Dashcommand and had good reviews for $30. Should arrive Tuesday. Already downloaded the app. I’ll know more and will be able to post more information once the scanner arrives.

EDIT #2:

I replaced the fuel filter and cleaned the MAF sensor today and that didn’t seem to do much. I also determined that the throttle position sensor is bad (no change in engine performance when I unplugged it) so I went ahead and ordered a replacement. I also ordered an inexpensive distributor with decent reviews. It’s made of aluminum at least. I only ordered it to get me through until I can afford a good one. It will also give me a chance to get the hang of adjusting the ignition timing.

Edited May 20, 2018 by erockwell
Updated post with new info

bronyaur · May 22, 2018

Sorry, but I should have mentioned that the DashCommand app will not read the CMP offset unless and until you add the PID and pay an additional $10 for that feature. Go to the Data Grid and select "Add PID" at the bottom right of the screen. On the ensuing screen, make sure "Supported PIDs" is selected at the top, then select "General". At the top of the list you will see "CMP Retard (CKP/CMP Correlation)". That's the one you want. That's probably the only additional feature you'd ever have to add.

Make sure you raise the engine RPMs over 1000 to accurately read the CMP offset. Watch the reading change after you reach 1000rpm and you'll understand why this is important. The other thing to mention is that the original distributor came with a slotted hold-down that allows you to turn the distributor a half inch or so either direction. Every aftermarket hold-down I've seen does not have this slot which makes it impossible to properly set the CMP offset. Hopefully you have the original.

Edited May 22, 2018 by bronyaur
Added DashCommand PID info.

erockwell · May 22, 2018

Well that’s interesting for a couple reasons. I was checking out the app and I found the timing section, which had a readout in degrees, and I thought that was what I needed to refer to. I also looked into buying the PIDs for my truck and the app said there weren’t any available. My scanner is arriving today. Is this something I can only access once the scanner is connected? I’ll be starting the operation today and will update with any issues. Also, is there an easy way to verify that the engine is at TDC on the compression stroke before I put the distributor in? I saw a video that mentioned it’s easy to accidentally put it in while it’s at TDC on the intake stroke and I want to make sure I have everything just right.

erockwell · May 22, 2018

Well I think I found the problem. According to the scanner, it reads +31° at 2000 RPM. Hooked everything up in the parking garage when I left work, and I’m going to install the new distributor and set everything up as soon as I get home.

Colossus · May 23, 2018

Cool, keep us posted.

erockwell · May 23, 2018

Okay. So I installed the new billet aluminum distributor (42 bucks on amazon, but had no reference marks so it was kind of a pain), verified it was working, cleared the codes, and drove around for a while. Coming on the heels of the MFI upgrade I just did, the truck has never run better. The weird thing is that the CMP offset still hovers between +18 and +20 degrees, and rotating the distributor has no effect. I have no idea what’s causing this, and while I’m thankful that the truck is running like it should, I want everything to be dialed in perfectly due to all the time and effort I’ve put into it. Thanks a lot. I don’t think I could have gotten this far on my own.

bronyaur · May 23, 2018

Try to set the CMP offset with the engine running at 1000-1200 rpm. You really need to get it set at zero (or dang close to it). You will experience hesitations at highway speeds and they will gradually worsen.

erockwell · May 23, 2018

I’ll try again when I get home, but yesterday when I tried to adjust it, the readout stayed at +20 no matter which way I turned the distributor, and I used the idle adjustment screw to keep it at right about 1200 rpm. What could be causing this issue?

bronyaur · May 24, 2018

I don't think you'll see the reading change as you move the distributor. I turn the motor off, move the distributor, start the motor, raise the throttle above 1000rpm and that's when I see the CMP offset get updated. What I'm reading suggests a little different procedure (like raising the engine above 1500rpm), but the main thing is that the engine needs to drop below 1000rpm (or shut off), then raised above 1000rpm (or 1500rpm, depending on what you read) to update. I'm not sure I ever really paid attention to my actual engine rpms when updating the cmp offset. I typically start the engine and snap the throttle up and watch the numbers update.

Edited May 24, 2018 by bronyaur

Scott Martin · December 28, 2019

Any updates on this thread?

ameadows7 · April 5, 2020

On 5/24/2018 at 1:28 PM, bronyaur said:

I don't think you'll see the reading change as you move the distributor. I turn the motor off, move the distributor, start the motor, raise the throttle above 1000rpm and that's when I see the CMP offset get updated. What I'm reading suggests a little different procedure (like raising the engine above 1500rpm), but the main thing is that the engine needs to drop below 1000rpm (or shut off), then raised above 1000rpm (or 1500rpm, depending on what you read) to update. I'm not sure I ever really paid attention to my actual engine rpms when updating the cmp offset. I typically start the engine and snap the throttle up and watch the numbers update.

Man I just want to thank you for the help on this. I registered an account just to tell you.

I've been pulling my hair out trying to figure out what to replace on my 97. I had a misfire between 2300-2600 rpm or so. If you tried to cruise at that rpm range it would feel pretty jumpy. I replaced the cap and rotor and it seemed like the hesitation was more pronounced. I was getting P0141 for an O2 sensor, P0300 for engine misfire, and P0304 for cyl. 4 misfire. I have an O2 sensor on the way already.

I saw this thread did exactly what you said. I paid $10 for the dash command app and another $10 to add the Cam r e t a r d PID and went outside and warmed up the truck. The cam r e t a r d was at 11 degrees. I thought surely that had to be the problem after reading this thread. I loosened the distributor clamp and turned it counterclockwise in small increments and revved it to 13 or 1400 rpm after each adjustment to refresh the cam r e tard reading. I finally got it to -0.2 degrees and let it idle for a few minutes after that. I turned the truck off and got back in the truck and cleared the codes with the same app.

Drove it down the street at a normal pace and held it in the previously problematic rpm range and it is running beautifully. It idles better than ever and feels so much smoother getting up to speed. I wonder how long my timing has been off like this.

Thanks again man, you were a huge help.

I had to put spaces in the word r e t a r d because my first post was automatically edited because apparently that's a bad word? Context is key...

Edited April 5, 2020 by ameadows7

bronyaur · April 5, 2020

Thanks for the feedback, and glad I could help.

99Tahoe4Dr · May 13, 2021

On 5/21/2018 at 11:45 PM, bronyaur said:

Sorry, but I should have mentioned that the DashCommand app will not read the CMP offset unless and until you add the PID and pay an additional $10 for that feature. Go to the Data Grid and select "Add PID" at the bottom right of the screen. On the ensuing screen, make sure "Supported PIDs" is selected at the top, then select "General". At the top of the list you will see "CMP ****** (CKP/CMP Correlation)". That's the one you want. That's probably the only additional feature you'd ever have to add.

Make sure you raise the engine RPMs over 1000 to accurately read the CMP offset. Watch the reading change after you reach 1000rpm and you'll understand why this is important. The other thing to mention is that the original distributor came with a slotted hold-down that allows you to turn the distributor a half inch or so either direction. Every aftermarket hold-down I've seen does not have this slot which makes it impossible to properly set the CMP offset. Hopefully you have the original.

I have a 99 Tahoe with the 5.7l and I purchased Dashcommand plus the PID for my truck but I cannot get the CMP values to show up at all. When I click 'add pid' CMP does not appear in the supported Pids but only in the all PIDs.

What can I be overlooking in the app? Are there other important settings that need to be set first?

Thanks