85IROCzzz

iTrader: (1)

I'm currently working on a '75 Jeep Truck that has a 350 TPI swap and I'm trying to get it running right. It has been sitting for a VERY long time (~10ish years) and I have it running pretty good now but there is still something not quite right. At first it was backfiring very badly out of the intake which I partly attributed to bad gas. I filled it with about 5 gallons of fresh 91 octane after draining a lot of very smelly bad gas that smelled like varnish, and now it starts great and sounds much better when you give it gas but still has an intermittent backfire out of the intake. Specifically once it starts to warm up it gets much worse, especially if it is in gear and you try to give it some gas, it hesitates badly and sometimes backfires. I connected it to tunerpro and was watching the data and I think the MAF may be suspect because it was around 12 g/s when the truck was trying to idle (had to keep it alive around 800-900RPM.) Is 12 g/s too high of a reading at idle? Also something I thought was very suspect was if I unplugged the MAF it ran exactly the same and the data kept reading like normal in tunerpro with it unplugged. Around 12 g/s at idle and it still moved when I gave it gas to like 50-60 g/s when I would rev it. I shotgunned a MAF and put in a cardone reman and it was the same. I was thinking it could be a wiring issue with the MAF burn off relay and power relays so I probed them with the power probe and took notes.

With the key off:



With the key on:



Engine running around 900-1000 RPM:




Are my voltages correct here? I feel like I shouldn't have an odd intermittent ground on the blue(?) wire on each relay while it's running.

I remember seeing a post here about the correct voltages at the relay connectors but I've searched and can't for the life of me find them

Any TPI gurus please let me know what you think and where I should go from here.

I should also mention TPS voltage is .53 and sweeps to 4.6 at WOT. I have also checked timing is 6*BTDC with the EST wire disconnected. Also did a rudimentary vacuum leak check with carb cleaner and spraying it everywhere didn't change idle. No codes set in computer either.

Thanks for any help.

Tuned Performance

iTrader: (92)

I don't think 12gps at idle is abnormally high. I've typically seen 9gps at idle.
this sounds like faulty injectors ohm them when warm 15-16 ohms ideally no more than 1/2 variance but 1ohm is ok.
Idle (650-750 should be more like it )warm seems high like the tps is sticking or min idle is not set correctly.
Sounds like a lean miss
Just for reference but I don't think the maf is faulty.


B12 shows the expected voltage Koeo and idle.
Since your not getting a maf low error 34 and the gps rise with rpm looks like the maf is working.

85IROCzzz

iTrader: (1)

Thanks for the reply!

I forgot to mention that I checked the resistance on the injectors after it got nice and warm (180+ degrees) and they all came out to exactly 17.3 ohms.

I had it idling perfectly the other day after doing the IAC minimum air setting procedure but over the course of a few days it has gotten worse and now it wont keep itself idling, it just dies.

Looking at the attached wiring diagram it looks like the dark blue should never be grounded, but with the engine running I am getting a very erroneous ground signal with my power probe (not a solid tone, but a quick beeping tone.) Not sure if you're familiar with a power probe vs a multimeter but it has a different beep for battery voltage and ground and it was rapidly beeping the ground tone on the dark blue at both relays. Should I replace both relays to be safe?

I will look into the possibility of injectors but not looking forward to pulling the fuel rail...this truck is tall!

Yes the gps rises, but is it normal for it to still rise with the MAF completely unplugged? I was still getting "ghost" readings like it was still plugged in. Does the computer go into some kind of simulated MAF mode when it doesn't see one so the car will still run? And the readings plugged/unplugged were identical.

I'll have to probe the ecm at B12 and see what it's getting.

Tuned Performance

iTrader: (92)

I don't recall the maf gps going up when unplugged in lhm. Iirc the gps reading is based off of voltage feedback to the ecm. I'm sure with fuel in the rails for 10 or so years the filterbaskets and pointless are full of varnished fuel deposits. I purchased a 87 c4 very last year that sat for 10-12 years . The po tried to start it the fp drew too much amperage and melted the fp relay and caused the ops to leak. After I repaired the fp relay and put a fuel pump in it and fresh fuel I pulled the injectors. 6 of the injectors were stuck shut I had to soak them in injector cleaner then shock them with a full 12v to unstick them. Then ultra sonic cleaned them installed new filter baskets and pointless caps and flow tested them on a home made test bench and otc mv injector tester. The filter baskets looked completely plugged with green varnish. If you have old Multic injectors Southbayfuelinjectors have newer Bosch 3s for a great price.

85IROCzzz

iTrader: (1)

Thanks I'll definitely look them up. I've bought a set of bosch III's from Injectors Plus I think? They used to be on these forums a while ago...but I will check out Southbay.

I didn't really think of the fuel sitting in the rails and injectors and varnishing up. So looks like I should go ahead and pull the injectors just for good measures.

Thanks for your help I will update as I can.

85IROCzzz

iTrader: (1)

So I did some probing at the ECM and at B12 I was getting 1.68volts with key on engine off, and when I started it it went to .78volts. With the MAF unplugged B12 had 5v. Isn't the MAF supposed to be giving 2.5-4 volts back to the ecm?

Tuned Performance

iTrader: (92)

To me looking at the schematic if power comes from the maf relay to e of the maf .
how can B12 get power with the maf unplugged.
The .78 seems like it would be a higher maf gps reading.
I think the idle programming table goes up to 1.46v
.73 is 8.36gps so it's sounds like .78 is about right for idle.
I guess I'm not fully understanding the pinout from the fsm.
I don't know the voltage peramiters for code 34 maf lo , 33 maf high is set at 45gps and I think this is triggered just off idle if gps hit 45.
If there was a maf issue I would think you would throw a code.
Your findings seem odd though unless the ecm is sending a 5v reference to the maf and the maf is powered bye 12v at E .
It could be a bad ecm I'm really not positive.

85IROCzzz

iTrader: (1)

I am going to replace both the MAF power and burn off relays for good measure but I was also leaning towards bad ECM.

Here are a few screen caps of my data log with WinALDL (tunerpro was on a different laptop):
Idling with MAF plugged in:



Unplugged MAF and immediately took another screen cap, no change in MAF reading in datalog:

Tuned Performance

iTrader: (92)

The grams per second look alittle high for idle, it may have a aftermarket cam.

85IROCzzz

iTrader: (1)

According to the owner it is a crate engine that he bought as a long block and had the TPI setup from an 87 Camaro put on it. He couldn't tell me the internals as far as a cam but said it was rated at something like 310 HP. It definitely has aluminum heads but I don't know what size.

Would that explain higher than average MAF gps?

Tuned Performance

iTrader: (92)

I would guess the cam is why the gps are higher than 8-9 like a stock engine.
I went through a few datalog of tuning I have done. A zz4 had 6.5 gps at idle, a fairly built 450hp 383 had 17gps.

85IROCzzz

iTrader: (1)

Relays did nothing different.

I went ahead and pulled the injectors to check out how they looked. I'm pretty sure a few were leaking because as soon as I pulled the rail out of the manifold, I primed the fuel pump and checked them out, and several of them were wet with fuel on the tips. Not steadily dripping or anything, just slightly wet.

So I'll be ordering a set of bosch III's from Southbay tomorrow.