Challenge tow rig issues: 95 Suburban Edition

So, we're on our way down to the challenge (in conneticut) and we are having some problems with our 1995 Suburban tow rig. It has the 5.7l V8. The motor started surging, cutting out, and surging more. When you give it throttle, it seems fine, but on decel, it will stumble. You can maintain constant throttle and it feels like it slows down and then it fells like it will advance timing all on its own and will feel fine for a minute, and then doing it all over again. It has a erratic yet cyclical pattern.

It just had a complete tune up, timing is set correctly, and it still does it. Then, we threw a coil at it (it was original and this thing has 217,000 miles) and it made it worse. So we started disconnecting more sensors one by one, and when we disconnected the map sensor, it smoothed out. We threw a map sensor at it and it STILL DOES IT!

Right now we hit the road again with the map sensor disconnected and its doing ok, but we want to really fix it. I think its only working because its in limp mode. Check engine light never came on until the sensor was disconnected. What do you guys think?

fuel filter ? bad fuel ?

It doesn't seem to be fuel related because its running ok without the map sensor connected. If it was either one of those it would be still doing it now.

95? Vacuum leak on and off, that's causing the map sensor to go from lean to rich, and back again? Add some brittle (arcing, grounding) wires from age, and I'd say you're lucky. Tomorrow, no rain, take a LONG, HARD look around the distributor, because after that, it's rain all the way to FL. That, or your fuel filter is clogging or the fuel pump is dying.

All the wires look fine. No vaccum leaks anywhere. Plug wires are brand new. It can't be a fuel issue, because it would be exhibiting the same issues with the map sensor unplugged. Maybe something to do with it being in closed loop right now?

Are you guys rolling or stuck?

Failing fuel pressure regulator? Can you borrow a fuel pressure testing gauge? If so, try to watch the fuel pressure as it starts to act up.

Check all of the vacuum lines, one at a time, for holding vacuum.

If you have a vacuum pump, try to apply vacuum to the MAP sensor and watch the voltage as the vacuum changes. It should go from 0-5V.

Carefully spray a little flammable liquid around the intake manifold at idle, see if the idle changes. Could have a leaking intake gasket or fitting.

In reply to turboswede: That's what I was thinking, too.

I live in CT, and if you're stuck near Old Saybrook (Rt. 9 and I-95) or theres-abouts, post, and I'll grab some stuff, and see if I can come and help you out. --Bob

We have made it to NY and the truck actually is running great. When we stopped at autozone I tested what I could included removing vacuum hoses, wiggleing connectors, tapping sensors, dumping water on everything. We replaced the coil because it was showing signs of shorting around the sides.

Now I hate just throwing parts at stuff without making sure it will solve the problem. We disconnected a few different sensors and the only thing that made it drivable was unplugging the map. Now this should put it into open loop and some kind of limp mode. From what I was reading fuel and timing will be based off tps. The fact that it runs great without a miss or bog makes me think there is enough fuel. I'm open to the idea of low fuel pressure but its not something we are able to test at this time. Next stop I'm going test resistance of the coolant temp sensor and o2 voltage.

Anyway I'm still open to ideas and thanks for all your input.

Sadly, i do not really have any specific advice to offer, but hopefully something will work out for you guys. I'll just bump the thread to keep it near the top. Since you ruled out a lot already, i will vote for coolant temp sensor, mine came unplugged on the saab once and the symptoms were not disssimlar. Good luck.

If the CEL is still on, grab a paper clip and jump the two terminals on the OBDI connector (should be the two on the upper right of the connector, top row) count the number of times the light blinks. It should show the MAP sensor code, but it might show any other codes too.

Depending on the build date of the truck being a '95 and all, it might be OBDII.. have 'em scan it.

wonder if it could be a bad O2 sensor?

I'll be heading down 95 in a few hours from LI, I'll bring my OBD scanner and keep an eye out for you on the road. I'm driving a white F550 with a white 40 gooseneck enclosed trailer.

MisfitToys HQ is just off of the NJTP south of Philly, we are leaving tomorrow about 7. If you need anything in our area or when we are behind you on the road, let us know

Chris 5 oh 8 Five 66 Sixty5 4Five

Awesome that's guys. We are actually taking 81 south so we can skip NY. We will be back on 95 near Richmond.

We stopped for fuel and I tested the coolant temp sensor and got 247 ohms which looks to be in spec. If I unplug it the truck will not start or run as all. As for the o2 I slid under to take a look and its original with 217 k on it. Everything around it was too hot so I could not get to the connector. Tps also tested fine. Oh well looks like we are going to keep rocking alpha-N for now.

As Sonic said, we'll be rolling out and heading down 95 about 8 tonite so if you need something give a holler (I'll pm my cell number) and we can try and catch up with you on the road.

I'd give a vote to that O2 sensor as well. Do yourself a favor, next stop get some penetrating oil and start soaking it now, just spray it every time you stop, sure it will smoke a little but if its been in there awhile its gonna be a bear to get out.

I'm thinking either a fault in the MAP sensor wiring or ECU myself, but it's also possible the ECU is ignoring another sensor in limp home mode and that sensor is bad.

I've had an issue with a leak in between the MAF and the throttlebody (idiot mechanic left flex boot off of TB) that made it run like E36 M3. But that was a MAF. Since the MAP should be reading manifold pressure, then anything in the intake tract seems unlikely.