Well, thought I would post up a quick summary of what I’ve done to try and make my 1995 Suburban run like it should.
I bought it with a bad fuel pump, but it would run with gas poured into the intake..hey it was cheap and I have done a few fuel pumps over the years. It had a rebuilt transmission put in about 2 years before I bought it though it had pretty much sat since then with a bad fuel pump.
I changed the fuel pump and the wiring harness in the tank and moved the ground for the pump to the main chassis ground at the rear of the suburban so it gets the full battery voltage. I checked the fuel pressure at the connection at the tank and it is about 11.5 psi(9-13 psi considered good).
The Suburban of course still had a misfire after changing the pump, plugs, wires, cap and rotor. It would actually misfire and stall if I tried giving too much throttle with any kind of load on the engine. I got a little frustrated and took it for a good thrashing and came back reliably missing and with a bit of a tinking sound. Did a compression check and found cylinder 7 had little compression. Took off valve cover and found intake pushrod on that cylinder bent and the valve stuck closed.
I rechecked the compression and it was all within about 15 psi(the high was 165 and the low was 140(cylinder 7)), checked the timing as per the manual and set it for 0 degrees BTDC with bypass wire disconnected. Checked the coolant temp sensor, sensor grounds, backprobed the TPS sensor to make sure it didn’t have any flat spots or jumps in it, and checked for worn timing chain set by turning the engine one way and then back the other until the rotor moved(found about 7 degrees of slop with 140k miles on the motor)..hooked up a vacuum gauge to make sure vacuum was good(17 at idle, drops with throttle blip, and comes back to about 24 as it settles back). So I would assume the mechanics of the motor are ok.
I thought this was the golden ticket to a good running vehicle, but even after I freed up the sticky valve and put in a new pushrod(verified as best I could that it was moving the valve as much as the other) it still had an intermittent miss with a backfire through the intake if you gave it too much pedal while under load and the Suburban would actually die, but restart without any trouble.
I then found golden ticket number 2, or so I thought, the distributor had way too much endplay in the shaft that allowed the rotor to go up about 200 thousandths and turn about 15-20 degrees or so…I ponied up the cash for a reman distributor and the suburban ran better, but still not correctly. Just for the pleasure of it I changed out the ignition coil as it was the only thing that I hadn’t changed out in the ignition system. Reman distributor number one still had more endplay than I liked so I traded it for a distributor that was actually within factory specs for the suburban, still not running good, but maybe a little bit better.
Now I started getting serious and bought an aldl cable for obd1 and got the software that I needed to see what the computer thought was going on with the motor and got a few disturbing pieces of data that has got me thinking what came first the chicken or the egg.
Here are a few screenshots of the interesting tidbits that I noted:
Will have to check that one, but here is what I am seeing in that one is that timing is jumping around at idle as it should and goes up jus t a bit as I added a bit of throttle, but the O2 sensor went lean initially then jumped rich and lean as it should, but the map value didn’t do as much as I thought it would as I let the rpm steady out.
Here’s another one that shows even more interesting stuff:
In this one I’m letting the truck idle and blipping the throttle a little bit which makes the map value go down as it should(higher volts = less vacuum) as the air rushes in and then rebounds to slightly higher than idle as the throttle is closed shuttind down the rush of air like it should, but for some reason the spark advance value goes down as the engine is trying to pick up rpm. I was thinking this was maybe one of those things that those computer geeks did to keep the engine from being overrevved in park, so I tried the next capture.
Hmmm…now aren’t those vertical lines in the spark advance value interesting. This screenshot was the suburban in gear brakes applied to load the motor and stabbing the throttle. On the first blip of the throttle it was only about 1/2 throttle, and as you can see the map value goes down to almost atmospheric as the air rushes into the engine, the O2 readings go rich and the spark advance approaches 0 degrees and then does that cool vertical line stuff. The engine popped and almost died, but continued to run after the first blip with rich O2 readings and not quite as low as before vacuum.
Just to highlight the issue I gave the suburban full throttle and it popped and died with similar values to the 1/2 throttle except for the engine not managing to stay running shown by the high map value.
Since I did these datalogging sessions, I have changed all the vacuum lines, swapped out MAP sensor with known good one, made a block off plate to seal off EGR to eliminate anything funky there, seafoamed the engine through the vacuum lines, ran about 20 gallons of gas through the motor limping the Suburban around town and to work. I’ve pulled the plugs and all seem to be nice and light tan like you would expect.
It still stumbles at idle, backfires through the throttle body if you give it too much throttle with a load on it. It doesn’t like overdrive at all because once it locks up the converter it loads the engine too much and causes the Suburban to lurch and surge very badly. I’ve been nursing it around in 3rd gear to keep the converter from locking up and keeping the rpms up on the highway though it does still buck a bit and doesn’t accelerate very well on hills at all and actually usually loses speeds even on slight inclines.
I’m planning on checking the valve action again to make sure everything is fine there and recheck the compression to double check that the numbers that I got before are still the same. Other than that I’m thinking that I’m going to have to try a reman computer to see if it is any better. It just is not a good thing that it’s only 3 degrees outside 
