Dieseling sometimes when hot

[chevytech87]

Hey all,

I’ve been driving my truck this week to break in the new motor and get a feel for the sniper EFI. Overall, been pretty happy with everything! I have noticed that sometimes, when the engine is at operating temp after a decent drive the truck “diesels” for a second or two. I’m thinking this may just be an idle screw/ IAC issue? First time I’ve tried to use this system and it’s a lot to take in. Any suggestions?

 

[Trucksareforwork]

I always assumed an efi wouldn’t diesel because the injectors turn off. No fuel = No dieseling.

I guess it could be your throttle plate or a vacuum leak.

 

[R Carnella]

Leaking injector maby,combined with not enough octane?

 

[pnwnvrdn]

Pull/inspect spark plugs, they will let you know how efficient they are performing...I'm of the belief you will need to go to hotter, or colder plug, determined by what you find on your inspection of the old ones.

 

[fast 99]

usual suspects

idle too fast, leaking injector, too hot, low octane fuel, vacuum leak, lean

 

[chevytech87]

usual suspects

idle too fast, leaking injector, too hot, low octane fuel, vacuum leak, lean

CTS shows ~183, I am running 87 right now, so I suppose that could be an issue. How lean is too lean at idle? I think I’m sitting around 13-14:1. Idles at about 800

 

[fast 99]

800 seems high unless it has a big cam requiring it. Automatic around 600 in gear is in the ballpark and 650 or so for manual. If there is an emissions sticker specs should be on it. Back in the day when we were having all sorts of drivability issues from low octane fuel, we adjusted the idle as slow as the customer could stand.

Automatics can be shut off in gear then put in park to minimize dieseling. If this problem continues an idle solenoid is an option. However, you need to do something NOW. Dieseling can very easily destroy a motor.

What octane fuel is in it?

 

[Bextreme04]

CTS shows ~183, I am running 87 right now, so I suppose that could be an issue. How lean is too lean at idle? I think I’m sitting around 13-14:1. Idles at about 800

That's a big spread. 13:1 is way richer than 14:1. If you are running 10% ethanol 87 octane, you should have a target AFR of 14.1-14.7:1. If you are running non-ethanol, you want target idle AFR to be 14.7:1. Cruise idle should be similar. Full throttle AFR should go into power enrichment and be ~12.5:1. Partial throttle AFR(but not high vacuum cruise) should be in the 13-14:1 range.

 

[fast 99]

If air pump equipped AF ratio will be skewed.

 

[Bextreme04]

If air pump equipped AF ratio will be skewed.

He says its a new motor(I'm assuming a crate motor) and is running a sniper EFI. I'd be shocked to see someone put a sniper EFI on and then keep the stock SMOG equipment, BWTHDIK

Also.. we need pics in this thread before Rusty comes up in here and throws a fit.

 

[fast 99]

He says its a new motor(I'm assuming a crate motor) and is running a sniper EFI. I'd be shocked to see someone put a sniper EFI on and then keep the stock SMOG equipment, BWTHDIK

Also.. we need pics in this thread before Rusty comes up in here and throws a fit.

Agree, just tossing it out there. Some areas still smog check.

 

[R Carnella]

Put some 93 in there and i suspect the dieseling will stop.

 

[chevytech87]

800 seems high unless it has a big cam requiring it.

What octane fuel is in it?

That's a big spread. 13:1 is way richer than 14:1. If you are running 10% ethanol 87 octane, you should have a target AFR of 14.1-14.7:1. If you are running non-ethanol, you want target idle AFR to be 14.7:1. Cruise idle should be similar. Full throttle AFR should go into power enrichment and be ~12.5:1. Partial throttle AFR(but not high vacuum cruise) should be in the 13-14:1 range.

@fast 99 I agree about 800 being high, I dropped it to 650 today and it’s seemingly much happier. The cam is substantial (Hydraulic roller 236/252' at .050" with 565" lift on 110'LSA) Had the idle set high for some reason I can’t remember when I was troubleshooting startup issues and never changed it. I am running pump 87, which I’ve been told I should stop doing. Will it run? Sure, but I’m guessing I’ll have more and more issues and I’m assuming it’ll spark knock at rpm once I break it in and can really get on it. So 93 it is from now on I guess. @Bextreme04 thanks for the info, I’ll have to check my tables and see what I’ve got desired set at. And suggestions for 93 AFR or are the numbers similar? For reference this is a freshly built 383 with some decent pistons and dart iron eagle 200 heads. Nothing too wild, but definitely not stock.

 

[chevytech87]

Also.. we need pics in this thread before Rusty comes up in here and throws a fit.

I promise i’ll take some tomorrow while I’m working on it! Haven’t taken any since I started driving it home again. It’s looking pretty sharp.

 

[fast 99]

@fast 99 I agree about 800 being high, I dropped it to 650 today and it’s seemingly much happier. The cam is substantial (Hydraulic roller 236/252' at .050" with 565" lift on 110'LSA) Had the idle set high for some reason I can’t remember when I was troubleshooting startup issues and never changed it. I am running pump 87, which I’ve been told I should stop doing. Will it run? Sure, but I’m guessing I’ll have more and more issues and I’m assuming it’ll spark knock at rpm once I break it in and can really get on it. So 93 it is from now on I guess. @Bextreme04 thanks for the info, I’ll have to check my tables and see what I’ve got desired set at. And suggestions for 93 AFR or are the numbers similar? For reference this is a freshly built 383 with some decent pistons and dart iron eagle 200 heads. Nothing too wild, but definitely not stock.

Those 2 items [RPM and octane] are likely enough to cause dieseling.

As much as it hurts, I use E free or high octane in the SB. Depending on the year today's high octane is actually lower than regular used to be. Everyday 30 cents a gallon regular was 94 or higher until they started to take the lead out.