1999 Ford Explorer with Miss at Low Speeds

[bretman1]

I have a 1999 Ford Explorer that we purchased a year ago, that I'm trying to get running properly. It had numerous issues, but the final major one is that it has a miss at low engine speeds. At speeds at 1500 rpm and above, it's runs smoothly, but below that, it's pretty rough. I pulled and cleaned the injectors (as well as running some Sea Foam and Lucas Cleaner in as I filled up a couple times). I've replaced the EGR valve (which is now unplugged), IAC valve (AC Delco), MAF (BWM), O2 sensors (Bosch), crank sensor (BWM), Cam sensor (just the pickup -- AC Delco), plugs, wires, coil pack (Delphi). I've also ohmed out the wiring harness to the various sensors. I've also run a compression check (all within spec), and a vacuum check (about -18.5 to -18 "Hg at idle in park). I've also checked the crank sensor signal on the scope and it looks very clean with the missing tooth well-represented. I also checked the fule pressure (65 psig with the engine running, 58 psig with just the key on). I do intermittently get error codes for missing cam sensor signal and multiple cylinder misfire. My next step is to scope the crank and cam signals simultaneously since it could be a sloppy timing chain. The PCM is also still on the list. It feels like an electronic problem (or perhaps the PCM switching modes) since the problem comes on at a specific engine speed range -- even when accelerating, you can feel misses until the engine speed gets above 1500 rpm. Any guidance would be greatly appreciated.

 

[anton831]

Sounds like it jumped a tooth on timing but I could be wrong.

 

[bretman1]

That is possible. I put a digital scope on the crank and cam sensor signals. Both are well-behaved, but I'm surprised the PCM can do anything with the cam signal -- it's very "sluggish" and takes more than a full rev to finish. It's within the AC size range specified in the Ford shop manual, but the manual doesn't say anything about timing relative to the crank signal. It's a variable reluctance-type pickup. It looks like the PCM must be looking for the initial rise on that signal, but the cam signal could easily by off 4-5 teeth on the crank, at least from what I see visually relative to the missing tooth on the crank signal. Does anyone happen to have a reference trace?

 

[2000StreetRod]

camshaft position sensor

On the SOHC V6 the camshaft position sensor only notifies the PCM if cylinder #1 is on the compression stroke or the exhaust stroke. It has no impact on timing of the ignition or fuel injector. The PCM relies on the crankshaft position sensor for timing.

What spark plugs did you install? What gap did you use? What did the ones you replaced look like?

 

[bretman1]

AC Delco Platinum's. As I recall, the gap was 0.052". The original plugs all looked normal except for cylinder #6 which had signficant Carbon buildup, which is why I pulled the injectors to flow them. I've put about 720 miles on it since changing the plugs, and they are running clean as I pulled them a couple days ago. The shop manual stresses the need for using an alignment tool on the cam speed pickup when installing one, which made me wonder if they're actually using the cam signal to define TDC on cylinder #1 on the crank signal (otherwise, the alignment really shouln't matter that much as you mention). I cleared all error codes after I started it this morning and ran it in to work. The check engine light never came on, but after I shut it off I scanned for error codes, it showed a DTC count of 1, but when I scanned for the codes, it showed none were stored. After I started it again the check engine light came on aftetr about 10s. The DTC count was still 1 and the infamous P0340 was present. That makes me think I'm intermittently losing the cam signal which is confusing the PCM, and I'm losing the injection and spark timing until the signal re-appears.

 

[2000StreetRod]

Which engine?

. . . The shop manual stresses the need for using an alignment tool on the cam speed pickup when installing one, which made me wonder if they're actually using the cam signal to define TDC on cylinder #1 on the crank signal (otherwise, the alignment really shouln't matter that much as you mention). . .

Since you mentioned that the camshaft position sensor (CPS) was a variable reluctor I assumed that you have the SOHC V6 which requires no alignment. I think the OHV V6 and the V8 use a hall effect CPS which requires an alignment. Which engine do you have?

 

[bretman1]

It is an OHV -- I called it a variable reluctor since the engine control manual defines the difference as a two-wire (variable reluctance) vs a 3-wire (Hall-effect) connector. It is a two-wire connector, but it's probably just a different style of Hall-effect sensor that Ford started using. The pickup assembly has the same body style as a distributor and definitely requires alignment if you remove it from the block. I was going to buy the alignment tool to ensure the alignment is correct, but since it doesn't appear that it's ever been moved, I took it to the Ford dealership yesterday to have them put it on the scanner.

 

[1Aauto]

welcome to the site

 

[bretman1]

And the scan indicates ... change the cam synchronizer. They said the cam sync signal bit was intermittent at all times. I suppose it could also be the PCM, but I'll start with the synchronizer and go from there. I should be eligible for Rock Auto platinum status soon...

 

[bretman1]

The cam synchronizer was completely off target -- about 120 degrees off, so obviously someone had moved it at some point. In any case, the bearing inside it was shot too, so I replaced it with a Motorcraft unit. Now it's smooth as silk.