My 95 K2500 Suburban has a 454 4L80e and 4.10s . It recently jerked violently backfired and stumbled. I was able to limp it home about 5 miles. I found the button in the dist cap crumbled. I replaced it with a new rotor also. It still runs horribly. It’s running rich with black smoke from the exhaust. I watched live data with my scanner but I’m not seeing anything that’s too far off. ButI’m missing something here. I did fire the parts cannon at it with no avail. My fuel pressure is at 29lbs. The fuel pump and injectors are new. Along with new a MAP, 02 and TPS. I’ve also replaced the module in the dist. The timing is at 4 deg with the wire unplugged. I’ll try to post pics of my scanner live data. What am I missing??
95 454TBI stumbling
- Thread starter wb292
- Start date
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someotherguy
Truly Awesome
Are all those new parts you mentioned replaced in the attempt to solve this new issue?
My #1 suspect would be the CTS but I see on your data that it's showing 178F coolant temp so that pretty much rules it out. Running so rich that it's smoking black and stumbling is quite often a completely failed CTS, but you'd see a ridiculously cold reading from it, not the 178F you show here.
#2 suspect is bad connection to the MAP sensor, I haven't seen too many bad MAP sensors, but the vacuum connection usually gets loose - rubber boot cracked, or the plastic tube broken. A false low vacuum signal from the MAP can cause a bad rich condition.
Fuel pressure sounds OK but is that at idle or under a load? The new injectors, are they 100% sure the correct ones for the 1994-1995 7.4 TBI which operate at the higher pressure spec (26-32 psi)
Richard
My #1 suspect would be the CTS but I see on your data that it's showing 178F coolant temp so that pretty much rules it out. Running so rich that it's smoking black and stumbling is quite often a completely failed CTS, but you'd see a ridiculously cold reading from it, not the 178F you show here.
#2 suspect is bad connection to the MAP sensor, I haven't seen too many bad MAP sensors, but the vacuum connection usually gets loose - rubber boot cracked, or the plastic tube broken. A false low vacuum signal from the MAP can cause a bad rich condition.
Fuel pressure sounds OK but is that at idle or under a load? The new injectors, are they 100% sure the correct ones for the 1994-1995 7.4 TBI which operate at the higher pressure spec (26-32 psi)
Richard
Thanks Richard,
Yes all parts are new. Either AC Dielco or Delphi. The injectors are AC Delos 2172281 30PSI.I installed a fuel pressure gauge at the TBI . It was taken at idle. I can’t see it while driving.The MAP vac line and tee was replaced in August. The 454 has two vac ports for the MAP. One off of each bank is intake runners. I may have a new MAP wiring connector. If so I’ll swap it out. I live sort of remote so I stock a lot of spares at home. I have 4 OBS trucks so that comes in kinda handy. I’m starting to think of a vacuum leak under the intake. Maybe a bad intake manifold gasket from the backfiring?
The data shown was at about 3800 feet of elevation. Does my Map readings look good at my elevation?
Yes all parts are new. Either AC Dielco or Delphi. The injectors are AC Delos 2172281 30PSI.I installed a fuel pressure gauge at the TBI . It was taken at idle. I can’t see it while driving.The MAP vac line and tee was replaced in August. The 454 has two vac ports for the MAP. One off of each bank is intake runners. I may have a new MAP wiring connector. If so I’ll swap it out. I live sort of remote so I stock a lot of spares at home. I have 4 OBS trucks so that comes in kinda handy. I’m starting to think of a vacuum leak under the intake. Maybe a bad intake manifold gasket from the backfiring?
The data shown was at about 3800 feet of elevation. Does my Map readings look good at my elevation?
someotherguy
Truly Awesome
Are you sure you have the MAP connected to the correct port? On my '93 7.4 it's connected at the same place all the other TBI's I've worked on have it connected - the single port on the middle rear of the TBI unit. It's been more than 12 years since I owned a '95 7.4 so I can't recall where that one was connected but what you said about two ports for the MAP doesn't sound right to me. Do you have a vacuum tree installed in one of the 1/2" NPT holes on the intake or something? If so, is the port on the middle rear of the TBI capped off? Do you have a legible vacuum diagram sticker on the truck still, and what does it say?
EDIT - I don't have a whole lot of pictures of my old '95 but I did just dig up a full-size one where I'd pulled the passenger valve cover and it does indeed have a vacuum fitting installed directly into the intake back there near the heater hose connector, and looks to have a tee on there going to where the MAP sensor should live when the bracket is in the correct place (it's shoved up towards the front of the engine in this pic)
So, my apologies for any confusion on year-to-year changes on these trucks. Here's the '95:
And here's my '93; you can see the boot/tube hooked to the rear of the TBI right between the fuel lines (and the lump of JB goop putty some previous owner used to plug off the broken heater hose quick connect)
So with all that aside, I don't know about a popped intake gasket, but you could rule that out with the unlit propane torch test. I'd think it might be more likely to be a popped TBI base gasket.
Richard
EDIT - I don't have a whole lot of pictures of my old '95 but I did just dig up a full-size one where I'd pulled the passenger valve cover and it does indeed have a vacuum fitting installed directly into the intake back there near the heater hose connector, and looks to have a tee on there going to where the MAP sensor should live when the bracket is in the correct place (it's shoved up towards the front of the engine in this pic)
So, my apologies for any confusion on year-to-year changes on these trucks. Here's the '95:
You must be registered for see images attach
And here's my '93; you can see the boot/tube hooked to the rear of the TBI right between the fuel lines (and the lump of JB goop putty some previous owner used to plug off the broken heater hose quick connect)
You must be registered for see images attach
So with all that aside, I don't know about a popped intake gasket, but you could rule that out with the unlit propane torch test. I'd think it might be more likely to be a popped TBI base gasket.
Richard
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A vacuum leak would cause the fuel trims to go rich, not lean. Your long-term fuel trim is way lean, short term is lean-to-normal. O2 voltage is high, and showing "rich" exhaust. Rich enough to blow black smoke, which is killing the catalytic converter.
I assumed you were at a higher altitude than me based on the BARO reading.
Does the electronic spark advance work?
Any dripping from the injectors, (damaged O-rings, or faulty pintles) or leakage from the injector pod? That engine is getting excess fuel somehow.
I assumed you were at a higher altitude than me based on the BARO reading.
Does the electronic spark advance work?
Any dripping from the injectors, (damaged O-rings, or faulty pintles) or leakage from the injector pod? That engine is getting excess fuel somehow.
What would be a normal range for the fuel trims and O2? Does the Map readings look to be in a normal range?
The injector spray pattern looks good. No dripping or damage that I can tell. When I installed the new injectors I used a new install kit. All new O-rings, gaskets, ETC. I will recheck the timing and then plug in the timing wire to see if the advance is stable. Hopefully that’s it.
Midrange of fuel trims (short- or long-term) is 128. More than ten numbers either way is cause for investigation. Sometimes less than that.
O2 will vary (quickly) between high voltage and low voltage--"high" being about .9 volts, low being perhaps .1, more or less. .45 is about mid-range. The faster they switch between "above" .45 and "below" .45 the better.
Your scan tool shows the rate of change across the mid-line as "crosscounts", the number displayed is so high I can hardly believe it on a TBI. And as I said...the faster it switches, the better.
Probably. It'd be a little low around here, but you're higher-altitude.
A bit of an update. I noticed the wiring connector to the MAP was is poor shape. Broken wire insulation and exposed wire. I replaced it with a new connector and it’s running much better. Although there is still a rough idle.
someotherguy
Truly Awesome
It's not really in a spot that is any more prone to damage than the rest of the underhood harness, so you should probably give that whole thing a thorough inspection and I wouldn't be surprised if you found more.
Richard
