jakwi

Hi all, I'm hoping someone here will be able to point me in the right direction.

I have an 86 cj7 with a 5.3l vortec from an 01 chevy tahoe, totally stock. The conversion was done several years ago and has only recently developed this issue.

It only seems to happen when outside temps get above 80 or so and it is fully warmed up.

Normally it will idle rock steady at 600 or so rpm.

When it starts surging it will vary between 450 and 750.

Using torque and an obd2 adapter I can see that the o2 sensors are working normally. The MAF varies between 4 and 7 which could be cause or symptom and the timing advance varies as well, which could also be a cause or symptom. Everything else looks normal, with no codes.

Any idea how I could troubleshoot this without just throwing parts at it?

Thanks for the help

jakwi

I also want to mention that I have removed and cleaned the throttle body and iac with no change.

PowellSixO

iTrader: (1)

Have you tested your coolant temperature sensor, and cleaned you maf?

jakwi

I havent, but I will now. The coolant temp sensor obd reading matches the mechanical gauge, so I assume it's working correctly, but is there a preferred way to test it?

Thanks

98redorangeta

iTrader: (10)

Check your MAP sensor reading and spray some carb cleaner around the intake see if you have a vacuum leak somewhere

2xLS1

iTrader: (25)

Spraying carb cleaner on all the plastic around the intake manifold is just dumb. It's not good for the injector o rings either. People need to just quit saying that. Do a smoke test if you suspect a vacuum leak.

98redorangeta

iTrader: (10)

Really that interesting never had this issue before. How many cars have you seen issues with sprayin carb cleaner around the intake?

2xLS1

iTrader: (25)

None, because I don't do it. It's dumb. I have a smoke machine. It is about the most caustic thing you can buy in a spray can. I've also been rebuilding carburetors for 45 years and I know what that **** does.

98redorangeta

iTrader: (10)

Oh ok so do I need to go buy a new intake and injectors cause I did this on my car?

LilJayV10

iTrader: (39)

No...no you don't. Unless you use 4 or 5 cases of carb cleaner on your intake you aren't going to hurt anything.


If you want to check for a vacuum leak either
1. what your 02 mV and STFT and spray carb cleaner and see if either bank goes rich.

2. Take a bottle of water, drill a 1/8 hole in it and spray it down the intake and see if if the engine sucks the water in. If it does, you found your leak.

2xLS1

iTrader: (25)

Yea keep telling people to spray the equivalent of lacquer thinner on their plastic and rubber parts.

98redorangeta

iTrader: (10)

The water bottle is also a good low cost option.

Darth_V8r

iTrader: (4)

Dude, all you need is a cigar and a glass of whiskey.

Puff on the cigar and blow out into the intake through the throttle. And let the blade snap shut. After three or four puffs you'll see every leak.

The whiskey is to enjoy with the rest of the cigar

LilJayV10

iTrader: (39)

I've been doing this **** for a while now and I don't recall ever seeing that in any manual........

Well it should be!!!!

LilJayV10

iTrader: (39)

Don't worry I will. Like anything else you need to use common sense...

Please re read the second sentence in case you missed it.

All of us are happy you have a smoke machine, most don't. I do but still use carb cleaner or water most of the time.

A smoke machine won't always find a leak. The engine vacuum plays a big role in finding a leak compared to a smoke machine that will only put out 3-5psi of pressure.

SlasherVRGR

iTrader: (1)

Does the 5.3 have the stock truck intake? The intake manifold gaskets commonly leak on them, my lm7 truck has been surging for a year until yesterday when I finally decided to swap the gaskets. It was also +25 lean on the ltfts at idle. All good now, no surging and it's actually -3 to -4 rich at idle now.

jakwi

Hi all, well I'm happy to see that my issue has spurred such a dynamic conversation. I cleaned the MAF sensor this weekend with MAF cleaner. It looked clean to begin with, but I did it anyway.

I did have the Knock sensors replaced last year, which requires the intake to be removed and the intake gaskets to be replaced.

So how about propane to check for vacuum leaks? I don't have a smoke machine, and I don't smoke, although I do have whiskey. ha! But I don't think that will help.

I haven't looked at the MAP sensor yet, but I'll add that to my list.

thanks for the guidance!

Darth_V8r

iTrader: (4)

You can use propane. I've used WD40 also. In the end, though I've always had the best luck with smoke.

Here's a quick test for you though. Unplug the maf and see if it still does it. Or if it gets worse. without the maf, you should go into speed density mode, so if there are any little airleaks, they won't matter. The maf fluctuating is probably more an effect than a cause, but the maf readings are used for fuel calculations.

Are you cable throttle by any chance? I'm never sure which truck motors are which type. Possibly your idle set screw got accidentally tweaked if you are mechanical throttle.

2xLS1

iTrader: (25)

Yes an unlit propane or map gas plumbing torch would be better than any solvent in a spray can. It is near impossible to detect leaks on the bottom of the port with that method though. Smoke machine is still the best way to find intake tract leaks on modern EFI.

jakwi

I do have a cable throttle, but most of the time it idles fine. Today for instance it is in the low 70's and so you wouldn't even know I had an issue. It only seems to reoccur when it get's up into the 80s and the engine is thoroughly warmed up.

In any case I'll take a look at it and make sure it's set where it should be.

thanks