1991 F150 5.8L 168k

Here's the symptoms:

When the engine is up to temp (and even more-so now that it's summer and getting hotter) :

1) It will buck at highway (55-65 mph) and cruising speeds (25-35 mph), but not all the time. It's not restricted to just those speeds, I just wanted to define what I meant. To better describe it, I would call it a "hard miss" although I don't think it's actually missing.

2) Almost anytime I slow down for a stop, or to turn a corner (and hence, put more load on the engine), it will die unless I shift to neutral. Sometimes I have to rev the gas to keep it from dying, but other times it will surge and recover.

I don't have an RPM range for when these events take place b/c my tach is unreliable.

I've pulled the codes, and KOEO & CM come up 111 system pass.

The only KOER code I'm getting is 311: Thermactor air system inoperative (right side) and that is with a scanner, not the count CEL blinks method.

I pulled fuel pressure and vac readings this morning, again, with engine up to temp:

FP held steady @ 40 PSI w/ the regulator unplugged and the engine running. When I did hook it up, FP held steady @ about 32 PSI. However, I did notice that as engine speed increased (I was operating the throttle by hand), the fuel pressure dropped. This puzzles me, because I thought that FP should increase with engine speed, right?

The vac readings had me puzzled. I took the readings from the brake booster line. Initially the vacuum held steady @ about 17 in*Hg, and vac did increase as engine speed increased. However, as the engine continued to idle, the vac dropped to about 15 in*Hg. The only variable that I can account for here is engine temp rising. Also, the needle on the vac gauge would not remain steady- constantly "vibrating" with about 1 in*Hg of play. Is that normal, or a dead indicator for a vac leak?

So...that's what I've got. I don't really know where to go from here, except that once the motor cools down I'm going to pull the plugs. I did inspect the distributor cap and rotor last week and they're in good shape.

The only other info I have to offer is that last fall I had to change the rear fuel tank. It's the only tank I have. I'm curious if maybe I didn't get a good connection between the plug and fuel pump, and so it's making intermittent contact, but to me that doesn't make sense b/c this wasn't an issue immediately after changing the tank.

As always, thanks for the help!

 

I had the same/similar symptoms you described and system pass code 111, then CM code 327. It steadily got worst until it wasn't safe to drive on city streets. 2 caps had leaked all fluid onyo the PCB of the ECM and damaged the traces. I also had replaced my fuel pump 2 years prior and thought that was the problem. It may be worth an hours time to check the ECM.

Just a thought to help ya out.

 

Yeah, the thought had crossed my mind to check the ECM as well.

What's the PCB? It's probably obvious, I'm just not connecting the dots.

 

Well everything inside the ECM looks all well and good to me, but I'm not a computer expert, either....





Also, the plugs look good. Color is a brownish/tan, which I believe is normal.

...now what?

 

I can't tell from the pictures dbgrif. I can't enlarge for clearer detail. The PCB is the main circuit board itself. You would be looking at the capacitors for any bulging, crust on top of them, leaked fluid undrneath the caps. Also look for burns around resistors and capacitors.

My 300 would pass the KOER tests and never throw a code, that was why I suggested the ECM as culprit.

Can you upload larger pictures and you can do a search to look at other blown caps and burned resistors for these ECMs.

 

In that case, my ECM is in good shape. There weren't any of the things you described. I can't upload pics larger than 97 kb so that's the best I've got- though you might be able to get a closer look if you go into my profile and look in my album "Pick em up truck" b/c that's where I uploaded them to.

Thanks. I'll do some more troubleshooting tomorrow.

 

Just a thought: could the ECT sensor be the culprit, even though it's not giving me any codes for it? I'm curious b/c this only happens when the engine is up to temp and, as I said, it's gotten worse w/ the summer heat.

Also, I'm curious about my MAP sensor. I just remembered a couple of years ago, I'd had a similar problem (symptoms aren't exactly the same, but close) and a bad MAP sensor turned out to be the issue.

How can I test these sensors? Thanks.

 

The codes seem to be the only reason why i can not say you are having the same problem i did with my wifes truck. On her truck the rear pump was slowly going out, and the throttle position sensor died as well. The front pump had been dead for some years so all i could run on was the rear pump.

On her truck the engine would only idle if i removed two vacuum lines and even then it would idle so high the engine started to build up a lot of heat so i would not leave it on for long. When i was able to drive it, the truck would not be able to pass second gear or it would buck so hard you thought the rear end was going to come out from under the truck.

Once i had a little extra cash to throw at the project we replaced the front fuel pump, replaced the TPS, and re-installed the vacuum lines. After that the truck has not had any problems and even starts faster. Before it seemed like the fuel had to travel a long distance and i had to prime the pump several times before the engine would start so i guess you could take it as a sign that the pump was on its last leg.

 

Thanks for sharing.

I just want to clarify that I changed the TANK, not the pump. I kept the same FDM. And with the fuel pressure holding steady, both with the engine running and not running, I don't think this is a fuel delivery issue.

 

So it occurred to me that, since I pulled the computer yesterday to check the circuit board, it was reset. So I decided today to take it on a drive and see what codes came up.

KOEO: 111 System pass

CM
334: DPFE or EVP circuit above the closed limit of 0.67 volts
634: Transmission MLPS circuit out of self test

KOER
311: Thermactor air system inoperative (right side)
334

So between the 334 and 311, I think it's time to start investigating the emissions and air pump system.

What really puzzles me is that the code 634 came up in the CM, when according to the information I have (my scanner book and oldfuelinjection.com), that code should only be appearing in the KOEO or KOER tests. I wonder if this indicates a bad connection somewhere??

 

So I just finished testing the EVP and EVR. Here's what I found:

With engine up to temp, at idle, the EVP reads 0.95 volts with the EVR hooked up. When I unplug the EVR, voltage drops to 0.45 volts, which leads me to believe that the EGR is not fully closing at idle.

So, I moved on to test the EVR according to the methods outlined here: Fuel Injection Technical Library » EGR Vacuum Regulator (EVR)

On the mechanical test (unhooking the electrical connector), I read 2.5 in*Hg of vacuum at idle. According to the method, there should not be any vacuum if the connector is unplugged.

On the electrical test, however, the engine died when I grounded out pin 33, which means the RPM changed and the EGR valve should have opened.

Even though the electrical test was technically a pass, the mechanical test for the EVR was not, unless I'm misunderstanding something. So at this point, it appears that the EVR is bad and needs to be replaced.

I'll keep updating as I go, but if someone else has any suggestions about what I need to be chasing after, I'm all ears.

 

So I replaced the EVR and took it for another test drive. Here's the new round of codes:

KOEO: 111 System pass.

CM: 111 System pass.

KOER

173: HEGO sensor circuit indicates system rich (right side).
334: DPFE or EVP circuit above the closed limit of 0.67 volts. (GRRR! )

But, there was no code 311 this time!

I don't have any more time this weekend, so I'll have to pick up where I left off next weekend. I'll test the new EVR mechanically and electronically to see what happens. If it's good mechanically, then perhaps there's a faulty signal to the EVR due to a poor connection.

I will say that, at least on the test drive, it did seem to improve. I didn't notice any bucking or surging, and it didn't try to die when slowing down. But, that was just the test drive- we'll see how it's running by the end of the week.

Thanks to all who have currently contributed, and to those who will.

 

Thanks for the constant updates. Guys like you make it easier for the rest of us to troubleshoot our problems. Please keep us updated on what you find.

 

Just thought I'd bump this thread up and give some more knowledgeable folks a chance to pay me $0.02 in words before I start in on this issue again this weekend.

The truck has been having much the same trouble as the week has progressed. The dying issue is the most difficult one to deal with. I'll pull codes again tomorrow and see what's come up over the past week.

Thanks guys!

 

Alright, so I finally got the new round of codes pulled.

KOEO
121: Closed throttle TPS voltage higher or lower than expected.

CM
173: HEGO sensor circuit indicates system rich (right side)
334: DPFE or EVP circuit above the closed limit of 0.67 volts.
634: Transmission Manual Lever Position Sensor circuit out of self test.

KOER
121
173
334

So I definitely need to figure out what's going on w/ the EGR. Based on last week's tests it's just not closing all the way, and barring the chance that my new EVR is bad (which I'll test to find out), I'd say that it's receiving a faulty signal due to either corrosion or my computer is bad.

I'll check the vacuum off the EVR following the same procedures as before and see what happens.