Are we chasing a ghost?

Since November, our 1999 Chev. Astro 2WD has had a problem: It will sporadically run rough for a few seconds at highway speed. It will feel like driving against a strong wind, or driving over a lot of cracks in the pavement, sort of a bucking. Then it smoothes out and the "Service Engine Soon" light will come on. The light will stay on for a day or two, then go out. This happens sporadically, I can't determine any specific circumstances when the problem happens. It happens when I use cruise control and when I don’t; it happens up hill and down hill; it happens in wet and dry conditions, etc. The only condition that I have figured out is that it does it between 60-70 mph.
Our mechanic hooks up his scope, and all it says is "random misfire." He says that all 6 cylinders are misfiring, almost like it is running lean. He is stumped, he says something is failing, but he can’t figure out what. What we know it is not: fuel injector, fuel pump, EGR valve, fuel filter, because we have had all of those things checked or replaced in the 5 years we have owned it.
At one point this winter it refused to start, and the mechanic determined that the coil was arcing, so that was replaced. Again, it randomly misfired after a couple of days.
Then we suddenly lost all the coolant, and discovered that the intake manifold gasket leaked. That was fixed, and a new serpentine belt was installed.
Last month, while cruising down the highway, the engine suddenly lost power, dropped down to less than 30 mph, and became very loud. We had it towed to our mechanic, who said that something in the lower part of the engine broke. The engine had about 124,500 miles on it. We had a remanufactured engine (with 100,000 mile warranty), new oil coolant lines and water pump installed.
Don't ask why we are spending so much on the dumb thing. We like the seating and towing capacity of the van. Plus, we can't afford a new auto loan.
We thought that would be then end of the trouble. But again, it ran for a few days just fine, and then the misfiring started again.
HELP! What could be wrong?:banghead:

One critical item that the PCM does not monitor is fuel pressure. Even though you have discounted the "fuel injector, fuel pump, EGR valve, fuel filter, because we have had all of those things checked or replaced in the 5 years" the possibility exists that fuel pressure is the root of the problem. Your CPFI system requires a minimum or 61 PSIG fuel pressure to even start. I've seen a few start as low as 58, but not consistenly. However, once started, standing fuel pressure normally drops to 56-58 PSIG due to a vacuum signal being applied to the pressure regulator. I've seen some still run with pressure as low as 48 PSIG once started, although not very well.

One of the commonly overlooked problems with electric fuel pumps is the electrical connections. Pumps are very sensitive to voltage. On Astros with CPFI, there are four separate connectors in the pump circuit AFTER the fuel pump relay. This doesn't include teh relay itself and connections ahead of it. Any or all of these can be suspect. There is one connector just above and forward of the fuel tank. In addition to this, there is a ground connection in the same location that could cause poor connection. There is another connector at the top of the in-tank pump/sensor unit. There is another connector just inside the top of the in-tank unit, then there is a connector right at the pump itself.

This example was from an Astro that was actually running properly, yet still shows signs of poor connection and resistance:

http://72.19.213.157/files/PumpConnector.jpg


Further pressure problems can be caused by the fluid connection between the pump and in-tank unit outlet. Some models incorporated a pulsation damper in this part of the system, and they can leak after time. There was a TSB issued by GM (66-63-09) for this problem. This should have been checked and the pulsation damper eliminated by the technician when the replacement pump was installed.

And if you have replaced the fuel filter, but five years ago, it's past time to do it again.

Thanks for the information. Btw, the fuel filter was just replaced in January. I will have the connections looked at.

While reading other posts, I was reading about the CPI or sometimes called a spider. What I gather is that is where the fuel pressure regulator is located. Should I have that looked at? Is there a way to check it it is failing, or should we have it replaced?

Is all of this misfiring, although intermittently, damaging the catalytic converter? Is there a way to check that also?

Sorry to not know all this stuff. I know how engines work, but not enough to do the work myself. And I have already spent lots of money on this thing, so I want to get it done and over with soon.

The control valves, tubes, and injector poppets of the CPI system are frequently called a "spider", since that's what they resemble. The FPR is located right next to the control valve assembly under the plenum cover.

The function of the FPR can be tested by simply connecting a pressure gauge to the fuel line. No disassembly required. Unfortunately, since your problem is apparently intermittent, monitoring it at the time when the phenomenon occurs is going to be nearly impossible. Hopefully, your technician has already tested fuel pressure with the engine both not running and running.

You may want to try an AC/Delco distributor cap and rotor, do not use an aftermarket one. I chased this problem for 2 days looking at fuel pressure and delivery, egr valve, etc... only to discover that the 2 week old cap and rotor I installed was the problem even though it looked fine. When compared to the AC/Delco rotor, the aftermarket one was about an 1/8" shorter. Just another thing to look at.

Find a little spark scatter from that? Excellent information.

Must be spark scatter. Do you know how frustrating it is when everything checks out OK and you still have a problem. I'm just glad I didn't have to pay to have a mechanic find it for me. Can you imagine what that bill would look like? I have been trying to post this info to as many of the 4.3l misfire posts I can. If I can save just one person from the frustration I went through, then it is well worth it.

Your 90 could have either a VIN-Z throttle body or VIN-W CPI 4.3 V-6. Fuel pressure in a VIN-Z is only 9-13 psi, where the CPI is 41-45. A miss at speed like that could be so many things, I'd start with a tune-up if it's been over 50k since it's been done, since your year didn't use Platinum plugs. The EGR passages through the manifold are known to clog (especially on the CPI engines) and carbon pieces can dislodge and get stuck in the EGR valve which would also cause an intermittent miss.

Good point, Rich. I wasn't aware that 1990 was a "crossover" year. And FWIW, I thought the standing pressure on a TBI220 was 11-13 PSI, but I could be mistaken.

Knowing which system it actually is makes quite a difference in the diagnosis. IIRC, the TBIs used a vacuum EGR (positive backressure type) and the CFI/CPFI used a digital EGR.

BTW - This is a "spider":

http://72.19.213.157/files/CPFIInjectorAssembly.jpg

Although it may resemble a spider, any true arachno-aware person would recognize the lack of two extremities, thereby disqualifying it as a "spider", while the version used on a V-8 has the extra two "legs" and would qualify. Not to split hairs or anything....

This one even has dirty "feet."

Um, it's a '99 and as stated before should have at least 61psi fuel pressure. Just don't want any late-comers to get confused.

Thanks for getting us back on track. I thought Soozie32 mentioned that it was a '99, and that the mechanic had discussed the "spider", which would indicate sequntial CPFI.

Confused? Late? That's usually me...

One critical item that the PCM does not monitor is fuel pressure. Even though you have discounted the "fuel injector, fuel pump, EGR valve, fuel filter, because we have had all of those things checked or replaced in the 5 years" the possibility exists that fuel pressure is the root of the problem. Your CPFI system requires a minimum or 61 PSIG fuel pressure to even start. I've seen a few start as low as 58, but not consistenly. However, once started, standing fuel pressure normally drops to 56-58 PSIG due to a vacuum signal being applied to the pressure regulator. I've seen some still run with pressure as low as 48 PSIG once started, although not very well.

One of the commonly overlooked problems with electric fuel pumps is the electrical connections. Pumps are very sensitive to voltage. On Astros with CPFI, there are four separate connectors in the pump circuit AFTER the fuel pump relay. This doesn't include teh relay itself and connections ahead of it. Any or all of these can be suspect. There is one connector just above and forward of the fuel tank. In addition to this, there is a ground connection in the same location that could cause poor connection. There is another connector at the top of the in-tank pump/sensor unit. There is another connector just inside the top of the in-tank unit, then there is a connector right at the pump itself.

This example was from an Astro that was actually running properly, yet still shows signs of poor connection and resistance:

http://72.19.213.157/files/PumpConnector.jpg


Further pressure problems can be caused by the fluid connection between the pump and in-tank unit outlet. Some models incorporated a pulsation damper in this part of the system, and they can leak after time. There was a TSB issued by GM (66-63-09) for this problem. This should have been checked and the pulsation damper eliminated by the technician when the replacement pump was installed.

And if you have replaced the fuel filter, but five years ago, it's past time to do it again.
Thank you Blue Bowtie, I had our mechanic check all that you suggested, and low and behold, one of the connections was corroded! He had never thought o check online for ideas, so it was pretty cool. He didn't even charge me for the labor, because he had been searching for the problem for so long. Thanks again!

Thanks for the reply. It's always good to get some closure rather than wonder whatever happened.

It's usually the simple things.