5.7 TBI Advice Needed

[RustCollector]

8 years ago I bought my '88 Suburban with 179k on the clock. Smoked a little on cold start, but very characteristic of dried out valve stem seals and not really worrying to me. I topped off the tank right by the gentleman's house and drove the truck about 40 miles home. Topped the tank off again and was surprised to see it got 18mpg on the way home.

Then I did some basic "new" vehicle maintenance like swapping all the fluids, new plugs and wires, air filter, and fuel filter. After I finished I started driving the truck. A week into daily driving (almost all freeway) I rechecked the mileage and got 10mpg. Ouch. I figured it was because of a restricted fuel filter even though it didn't feel light on power prior to the swap. Anyway, I figured swapping it out brought it back to reality. 10mpg sucks, but feels more right for this rig than 18, so I just accepted it and drove on.

Fast forward a year of driving and the smokey startup became a smokey first 15 to 30 minutes and the smoke clouds were getting bigger and bigger. Ran a compression test and came back average 130psi. Believing I was past the leaky valve seals stage, I got myself a new block and got to work.

0.060" over bore, stock stroke with a forged crank, a relatively mild cam with a little more lopey idle, 4 angle valve heads with 1.95 intake 1.65 exhaust, roller tipped rockers, and reduced chamber volume (don't remember the exact numbers after 7 years, but I remember about a 9:1 compression ratio came out of the calculations. Nothing extreme, just a slightly bumped up motor. Attached it to 1-5/8" long tube headers with a dual 2.25" exhaust running Magnaflow cats and Cherry Bomb Turbo mufflers. Welded custom bungs onto the headers to keep the factory air pump and O2 sensor.

I also rebuilt the throttle body, replaced the regulator guts, and left the stock injectors that *looked* like they were flowing OK. I rebuilt the 700R4 with a full B&M kit, put a 1500 stall RV converter in it, rebuilt the NP208, and to TRY and help with my mileage I swapped out the original axles for 3.08's (I can't remember if the stock was 3.78 or 3.83). Dual electric fans on a new 4 core radiator, a high flow water pump, some custom autometer gauges, a B&M floor shifter, new automatic hubs up front...

Needless to say I got a little carried away. And for my $10,000... I dropped to 9mpg . I also developed an idle issue. It keeps wanting to stall at lights. I have to double foot it (hold the brake with the left foot while feathering the throttle with the right) to keep it from stalling. Doesn't take much feathering, but if I don't it I will be restarting it. It moves like a scalded dog when you leave the light, but it doesn't like the waiting part.

A couple years later after my daughter was born I had 2 tires blow out back to back, long story short the truck has been parked for 5 years. I need it back on the road now and I need to deal with my idle issue at the very least, and I'd like to do something about the rate at which that massive 40 gallon tank drains.

My knee jerk reaction is to replace the regulator guts again in the throttle body and replace the injectors... I mean that's the only part of the system that isn't pretty much new. But that's a chunk of change that could go elsewhere and I'm not confident there's even anything wrong with them. It's one of those last resort solutions.

Any suggestions on why she can't run a smooth idle on her own? Or why it sucks down a gallon every 9 miles?

 

[waterpirate]

Have you looked into the temp sensor in the intake manifold? I went down a similar road before toasting the engine. If the truck runs, pull the temp sensor plug off and look for any change. The puter has a limp home mode when plug is removed.
Hope this helps
Eric

 

[1987 GMC Jimmy]

I don’t think the 360 c^3 displacement is going to need updated injectors to adequately feed the motor versus when it was a 350. They can become dirty over time, and you can always have them flow tested, cleaned, whatever by someone who does that (e.g. WitchHunter Performance). There’s a buzz test you can do to make sure they’re not weak using a 9V battery, and you can ohm them out on a low scale to make sure they have the proper resistance, I believe you want 1.2, and if it’s less than 1, you’re not working right. Bad injectors will have a weak “snap” to them, and the resistance will read low, maybe around half of nominal.

Aside from blindly throwing parts at it and seeing what changes, there’s nothing you can do going forward without seeing the ECM serial data. You’d do both from startup to operating temp while in park so you can really study it, and if something’s really wrong it could show up here, and then driving around with it hooked up to see what, if any, issues come up. You’ll need to log it when driving so you can study it after the fact. The software is public domain, either TunerProRT or WinALDL, which I run the former. You can order a fancy, pre-made GM OBD 1 to USB cable, and that’ll run you about fifty bucks, or you can easily make your own for about five, give or take. There’s a sticky in the TBI/EFI section by user “Georgeb” detailing what all you have to do to get set up. It’s mostly going online, joining GearHead EFI, and downloading stuff from them, and I personally spent about $4 on my cable, and it either came from eBay or Amazon.

My guess is that the CTS may not be reading all the way up to operating temp so the computer just assumes it doesn’t warm up and holds it rich, or the MAP sensor is getting low/erratic vacuum readings because of the cam, especially at idle, and that creates a rough, low idle, and I wouldn’t be surprised if there’s occasional hesitation or the SES light comes on for a sec. If there’s any kind of exhaust leak proximal to the O2 sensor, that can skew how rich or lean the exhaust looks versus how it actually is, which could be another issue. If lockup isn’t engaging for whatever reason, that’s creating a lot of slippage/heat, therefore loss of economy on the highway. I could go on discussing how subtle issues can snowball, but a close perusing of the data would answer a lot of questions, rule out some variables, and put you on the right track to diagnosing what issues you have.

 

[bucket]

The best I ever got out of my '88 TBI 350 Suburban 4x4 was 11mpg. It was usually 10mpg. Get those horrid 3.08's back out of there and you can be back up to 10-11 mpg. That initial 18mpg you got was a calculation fluke.

Sounds like you may need to clean the passages for the IAC valve, set your base idle screw, and then check the adjustment of the throttle position sensor. You probably need to check that your IAC valve is actually functioning too.

 

[1987 GMC Jimmy]

I also thought 18 was too high, at least for an average, a tad high for highway. I meant to mention that, but I’m not feeling so hot the past few days. I’d do an IACV relearn, and IAC counts would be good to know. It likely doesn’t need too much to run properly, but I can’t stress the value of having serial data when you’re trying to diagnose engine management issues beyond rudimentary tuneup stuff, pulling codes, etc.

 

[4WDKC]

I didnt see anything about tuning the efi, that needs to be done for the new cam and remove the egr since you have headers. The check engine light on because of the egr adds fuel and pulls timing so you were wasting gas and no additional power. Also lopy cam creates slow air speed at low rpm and reduces low end torque. True duals also reduces low end torque. 308 gears and tall tires not only reduces performance it can cause you to use more fuel trying to get the big truck going.

 

[RustCollector]

I didnt see anything about tuning the efi, that needs to be done for the new cam and remove the egr since you have headers. The check engine light on because of the egr adds fuel and pulls timing so you were wasting gas and no additional power. Also lopy cam creates slow air speed at low rpm and reduces low end torque. True duals also reduces low end torque. 308 gears and tall tires not only reduces performance it can cause you to use more fuel trying to get the big truck going.

Elaborate for me...

How do I "tune" the EFI?

Why does the EGR need to come off? Headers help to scavenge the exhaust gasses, the EGR helps to cool the combustion so it can run leaner without burning too hot. If anything, the headers pulling more gasses out means the EGR is needed more. Less back pressure = less residual gasses in the chamber = hotter chamber (hence why the valves can melt w/ headers and too large of an exhaust). The EGR would counteract that effect. Unless i'm missing something, i'm not seeing this one... although i'm definitely open to the possibility that i'm missing something, so do tell if i am.

The check engine light is not on...

I'll buy the lopy cam slowing the idle air, but what do i do about it?

Low end torque feels good, but i did step the true duals down to 2.25" so it wasn't stupid open.

It doesn't have tall tires with the 3.08's. Bone stock. But my highway is only like 11 or 12, so still total crap, which the 3.08's should have improved... i thought.

 

[4WDKC]

Elaborate for me...

How do I "tune" the EFI?

Why does the EGR need to come off? Headers help to scavenge the exhaust gasses, the EGR helps to cool the combustion so it can run leaner without burning too hot. If anything, the headers pulling more gasses out means the EGR is needed more. Less back pressure = less residual gasses in the chamber = hotter chamber (hence why the valves can melt w/ headers and too large of an exhaust). The EGR would counteract that effect. Unless i'm missing something, i'm not seeing this one... although i'm definitely open to the possibility that i'm missing something, so do tell if i am.

The check engine light is not on...

I'll buy the lopy cam slowing the idle air, but what do i do about it?

Low end torque feels good, but i did step the true duals down to 2.25" so it wasn't stupid open.

It doesn't have tall tires with the 3.08's. Bone stock. But my highway is only like 11 or 12, so still total crap, which the 3.08's should have improved... i thought.

Brain fart, i was thinking there was connection from the exhaust manifolds to that intake/egr that headers dont have. Join www.gearhead-efi.com and read, only thing you can do about idle air speed is to buy a cam less lopey. The idea behind lower rpms equals better mpg is very common and partly true. 308s will get you better mpg than huge gears like 4.88+. Where it differs is load on the engine the higher the load the more gas you have to give it to get it going and keep it going. Performance gears lower load to get it going and keep it going, much like short and long handle ratchets short would be 308 and long would be performance gears.

 

[RustCollector]

Brain fart, i was thinking there was connection from the exhaust manifolds to that intake/egr that headers dont have. Join www.gearhead-efi.com and read, only thing you can do about idle air speed is to buy a cam less lopey. The idea behind lower rpms equals better mpg is very common and partly true. 308s will get you better mpg than huge gears like 4.88+. Where it differs is load on the engine the higher the load the more gas you have to give it to get it going and keep it going. Performance gears lower load to get it going and keep it going, much like short and long handle ratchets short would be 308 and long would be performance gears.

Cool. I'll check it out. Thanks!

You're thinking of the air pump tubes that go from the air pump, around the back of the motor, and in to ports on the exhaust manifolds at #1 and #2.

I actually kept the air pump, oddly enough. Just welded bungs into the #1 and #2 tubes to accept the pump tubes.