TBI Rough Idle when warm

[studigggs]

I've never used a Snap-On MT2400. What is "Power/Spark" referring to? I understand voltage required to fire the plug would be in the KV range, but I don't understand the terms "power" and "spark" in this application. Is "spark" the millisecond duration of the spark? In general, as voltage goes up, duration goes DOWN, which is the opposite of what your results are showing.

"Primary" and "Secondary" ignition are of course very closely related. A problem on the primary side can easily show up as a problem on the secondary side, too. However, some primary-side problems won't cause low output on the secondary side; in part because most folks look at voltage and largely ignore duration; and in part because some primary problems are more related to timing than to output power.


Depends on the reason for the misfire. A mixture too lean to burn would drive the voltage higher because air is less conductive than a burnable air-fuel mixture. OTOH, if the spark were grounding or the coil was weak, the KV would be low.


2-4-6-8 don't share "fuel". Like most V-8 vehicles with a "wet" intake manifold, the TBI uses a dual-plane intake system, where the outer two cylinders of one bank, and the inner two cylinders of the other bank share fuel.

So 2-3-5-8 share one injector, and 1-4-6-7 share the other injector. If you look at how the intake manifold runners are laid-out, you'll see how the injectors are shared among the cylinders.

A 2KV difference in firing voltage could EASILY be spark plug gap or condition. "I" would be looking at normal tune-up stuff: Plug condition, plug wire condition, cap/rotor condition, etc.

Yes, pickup coils can be a problem. I had a high-speed misfire on my '88, that got worse and worse (lower-speed misfires) until the truck stalled at a stop sign and wouldn't re-start. Pickup coil wires can break, or the magnets get weak; either one reduces or ends the signal to the ignition module.

This is great and THANK YOU for taking the time to educate me. Learning more each day. So the MT2400 calls firing voltage "power KV" and the KV of the spark line "Spark kV". There is a way to read burn time as well, but I did not have that screen up during this test.

My plan is to do the tune up first as it is time anyway. I'll also check the primary components as recommended by other posters.

 

[Redneckgeriatric]

this is a great thread! im gonna get one of them newfangled scanners! looks a lot easier to carry than an oscilloscope. haha

yall recommend a good scanner to look for (used) and can they work on pre obd, obd , obd2 and canbus, or do ya need different scanner for each protocol?
my ecm manipulation experience is toward the tuning, not so much drivability.

yall keep talkin, this is great info!

 

[Schurkey]

this is a great thread! im gonna get one of them newfangled scanners! looks a lot easier to carry than an oscilloscope. haha

A scan tool and an automotive oscilloscope do different things. They're both VERY useful tools.

'Scopes and ignitions have the issue that the manufacturers generally didn't provide "test points" in the distributorless systems for the primary circuit, and with coil-on-plug, you've gotta work to get at the secondary. So ignition oscilloscopes have very much gone out-of-style; even if the scope is still tremendously useful for electric motors (fuel pumps, HVAC blowers, electric fans, etc) and various sensors.

Scan tools are more familiar now. There's just no point to doing "driveability" work on a computer-controlled vehicle without one; even if it is possible and a DVOM can do most of what a scan tool does. The DVOM takes forever, one circuit at a time, even with a breakout box. The scan tool does it with updates from the entire data stream every second or even faster.

yall recommend a good scanner to look for (used) and can they work on pre obd, obd , obd2 and canbus, or do ya need different scanner for each protocol?
my ecm manipulation experience is toward the tuning, not so much drivability.

No such thing as a "scan tool" for "pre-OBD". Now you're back to the automotive oscilloscope.
My MTG2500 works on OBD, OBD-2 and has an adapter for canbus that I've never used. The vehicles it services are dependent on the software cartridge you slap into the main body. There is no VIN-specific software for the 2500-series newer than 2009, which was when Snap-On ended support for the 2500. They quit selling them long before that. I have software for "Domestic" (GM, Ford, Chrysler, Jeep) from 80 1/2 to '99 in one cartridge, and "Domestic" + "Asian" from '96--2006 in another. Then you need vehicle adapters and "keys". The point being, you can own a wonderful scan tool, but if you don't have the VEHICLE-SPECIFIC software and adapters for the vehicle you're testing, you're screwed. There are other brands, and newer models from Snap-On you should also consider. I've been threatening to replace my MTG2500 with something newer for the last ten years.

Better to get a used "professional grade" scan tool than a brand-new 'consumer-grade" tool. Some of the cheap tools won't read ABS, instrument clusters, transmissions, airbags, etc.

 

[Redneckgeriatric]

thanks for that, shurkey. im familiar with having to buy keys, or rights, or licenses, or whatever they call them. but that is with tuning software, which allows you to watch and manipulate and log all the pertinent thingy bobs.

so i guess if im gonna pay to play, its best to get tuning wares and use the logging ability vs the snap on stuff?

long time ago, ware named tweecer was available. used it on laptop in 1990 mustang. but after that, was sert for hardly davidson. many other wares were available with sharing capabilities.

you familiar with those?

these days, i just want to keep 2 vehicles running at prime. im old

 

[studigggs]

thanks for that, shurkey. im familiar with having to buy keys, or rights, or licenses, or whatever they call them. but that is with tuning software, which allows you to watch and manipulate and log all the pertinent thingy bobs.

so i guess if im gonna pay to play, its best to get tuning wares and use the logging ability vs the snap on stuff?

long time ago, ware named tweecer was available. used it on laptop in 1990 mustang. but after that, was sert for hardly davidson. many other wares were available with sharing capabilities.

you familiar with those?

these days, i just want to keep 2 vehicles running at prime. im old

I followed Shurkey's advice and picked up a mt2500 off of eBay. It boots up immediately and never seems to miss a beat. I have a Solus Pro with software up to 2016 on it that is nicer for the graphing capability, but takes a long time to boot, has a proprietary battery and, as I found out today, doesn't display data for my '90 truck correctly. The MT2400 graphing DMM is pretty cool because of the software. Every sensor in the GMT400 is in the database, so if you want to test, say, the MAP. You just select that sensor and it will give you step-by step how to test it. Even has pictures of the connector, directions on where to probe and even sets the DMM up for you with the correct scales and an example trace. For newbies like me its a real time saver.

 

[Schurkey]

im familiar with having to buy keys, or rights, or licenses, or whatever they call them.

No, the Snap-On "keys" are physical objects used with the OBD-2 connector. Key K-9 is the only one I've used, although my scan tool came with about a dozen of 'em.

so i guess if im gonna pay to play, its best to get tuning wares and use the logging ability vs the snap on stuff?

No idea. I've never "tuned" an on-board computer, merely "fixed" them by replacement of the faulty parts or wire harness.

long time ago, ware named tweecer was available. used it on laptop in 1990 mustang. but after that, was sert for hardly davidson. many other wares were available with sharing capabilities.

you familiar with those?

Never heard of them until your post.

these days, i just want to keep 2 vehicles running at prime. im old

I need to get off my ass, I have half-a-dozen that need attention, and I'm old, too.

 

[Redneckgeriatric]

I followed Shurkey's advice and picked up a mt2500 off of eBay. It boots up immediately and never seems to miss a beat. I have a Solus Pro with software up to 2016 on it that is nicer for the graphing capability, but takes a long time to boot, has a proprietary battery and, as I found out today, doesn't display data for my '90 truck correctly. The MT2400 graphing DMM is pretty cool because of the software. Every sensor in the GMT400 is in the database, so if you want to test, say, the MAP. You just select that sensor and it will give you step-by step how to test it. Even has pictures of the connector, directions on where to probe and even sets the DMM up for you with the correct scales and an example trace. For newbies like me its a real time saver.

thanks for that info! i was looking at those on ebay. to be clear, after the initial purchase, you can connect and use the mt2400 free of additional charges/fees?

 

[studigggs]

thanks for that info! i was looking at those on ebay. to be clear, after the initial purchase, you can connect and use the mt2400 free of additional charges/fees?

No annual fees. All the software is stored on the device. You wont have any support from Snap-On either if something does break as these are not supported any longer. You can buy replacement adapters and Personality keys direct from Snap-On, but as far as the actual scanner or DMM unit, there are no parts available save from eBay.

 

[Schurkey]

Yep...Orphaned.

 

[Redneckgeriatric]

thank yall for the info. looks like the best bang for the buck for a couple hundred bucks.
Merry Christmas